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Introduction to Tarski’s problem and Model
Theory
Abderezak Ould Houcine
Camille Jordan Institute, University Lyon 1, France
Non Positive Curvature and the Elementary Theory of Free Groups,Anogia, June 10, 2008
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A little Model Theory of Groups: Formulas & Sentences
Definition
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
• Formula φ(z), with free variables z,
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
• Formula φ(z), with free variables z,
φ(z) := ∀x1∃y1 . . . ∀xn∃ynϕ(x1, y1, . . . , xn, yn, z),
where ϕ(x , y , z) is a quantifier-free formula.
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
• Formula φ(z), with free variables z,
φ(z) := ∀x1∃y1 . . . ∀xn∃ynϕ(x1, y1, . . . , xn, yn, z),
where ϕ(x , y , z) is a quantifier-free formula.
• g ∈ G =⇒ φ(g) is a formula with parameters from G .
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
• Formula φ(z), with free variables z,
φ(z) := ∀x1∃y1 . . . ∀xn∃ynϕ(x1, y1, . . . , xn, yn, z),
where ϕ(x , y , z) is a quantifier-free formula.
• g ∈ G =⇒ φ(g) is a formula with parameters from G .
• Sentences := formulas without free variables.
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A little Model Theory of Groups: Formulas & Sentences
Definition
• Quantifier-free formula ϕ(x), with free variables x ,
ϕ(x) :=∨
1≤i≤n
(∧
1≤j≤pi
wij(x) = 1∧
1≤j≤qi
vij(x) 6= 1),
wij and vij are words on {x1, . . . , xn}±1.
• Formula φ(z), with free variables z,
φ(z) := ∀x1∃y1 . . . ∀xn∃ynϕ(x1, y1, . . . , xn, yn, z),
where ϕ(x , y , z) is a quantifier-free formula.
• g ∈ G =⇒ φ(g) is a formula with parameters from G .
• Sentences := formulas without free variables.
• G |= φ ⇔ G satisfies φ.
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A little Model Theory of Groups: Formulas & Sentences
Examples:
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
G |= φ(g) ⇔ g ∈ Z (G )
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
G |= φ(g) ⇔ g ∈ Z (G )
• φ = ∀x∀y(x−1y−1xy = 1),
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
G |= φ(g) ⇔ g ∈ Z (G )
• φ = ∀x∀y(x−1y−1xy = 1),
G |= φ ⇔ G is abelian
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
G |= φ(g) ⇔ g ∈ Z (G )
• φ = ∀x∀y(x−1y−1xy = 1),
G |= φ ⇔ G is abelian
• φn = ∀x(xn = 1 ⇒ x = 1), n ≥ 1,
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A little Model Theory of Groups: Formulas & Sentences
Examples:
• φ(z) = ∀x(x−1z−1xz = 1),
G |= φ(g) ⇔ g ∈ Z (G )
• φ = ∀x∀y(x−1y−1xy = 1),
G |= φ ⇔ G is abelian
• φn = ∀x(xn = 1 ⇒ x = 1), n ≥ 1,
G |= φn for any n ≥ 1 ⇔ G is torsion-free
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A little Model Theory of Groups: Elementary
equivalence
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
• G is a group,
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
• G is a group,
Th(G ) = {φ a sentence |G |= φ},
Th(G ) is the complete elementary theory of G .
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
• G is a group,
Th(G ) = {φ a sentence |G |= φ},
Th(G ) is the complete elementary theory of G .It is complete; that is for any sentence φ either φ ∈ Th(G ) or¬φ ∈ Th(G ).
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
• G is a group,
Th(G ) = {φ a sentence |G |= φ},
Th(G ) is the complete elementary theory of G .It is complete; that is for any sentence φ either φ ∈ Th(G ) or¬φ ∈ Th(G ).
• Γ =⋂
G∈TFH
Th(G ),
TFH:= class of torsion-free hyperbolic groups.
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A little Model Theory of Groups: Elementary
equivalence
• First order theory:= set of sentences.
Examples:
• G is a group,
Th(G ) = {φ a sentence |G |= φ},
Th(G ) is the complete elementary theory of G .It is complete; that is for any sentence φ either φ ∈ Th(G ) or¬φ ∈ Th(G ).
• Γ =⋂
G∈TFH
Th(G ),
TFH:= class of torsion-free hyperbolic groups.Γ is a first order theory but it is not complete.
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A little Model Theory of Groups: Elementary
equivalence
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
G |= φ(h) ⇒ H |= φ(h).
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
G |= φ(h) ⇒ H |= φ(h).
• (Tarski-Vaught Test)
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
G |= φ(h) ⇒ H |= φ(h).
• (Tarski-Vaught Test) If H ≤ G , then
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
G |= φ(h) ⇒ H |= φ(h).
• (Tarski-Vaught Test) If H ≤ G , then
H ≺ G
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A little Model Theory of Groups: Elementary
equivalence
• H and G are elementary equivalent ⇔ Th(G ) = Th(H).
A stronger property:
• If H ≤ G , H is an elementary subgroup of G , H ≺ G , if forany formula φ(z), for any tuple h in H,
G |= φ(h) ⇒ H |= φ(h).
• (Tarski-Vaught Test) If H ≤ G , then
H ≺ G ⇔
{
for any formula φ(x ; z), for any h ∈ H,∃c ∈ G ,G |= φ(c ; h) ⇒ ∃c ∈ H,G |= φ(c , h)
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A little Model Theory of Groups: Elementary
equivalence
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A little Model Theory of Groups: Elementary
equivalence
Roughly speaking:
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A little Model Theory of Groups: Elementary
equivalence
Roughly speaking:
Th(H) = Th(G )
m
H and G are indiscernible in the first order logic
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A little Model Theory of Groups: Elementary
equivalence
Roughly speaking:
Th(H) = Th(G )
m
H and G are indiscernible in the first order logic
and
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A little Model Theory of Groups: Elementary
equivalence
Roughly speaking:
Th(H) = Th(G )
m
H and G are indiscernible in the first order logic
andH ≺ G
m
H is ”algebraically closed” in G
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Tarski’s Problem
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Tarski’s Problem
FX the free group on X , |X | the cardinal of X .
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Tarski’s Problem
FX the free group on X , |X | the cardinal of X .
Around 1945, Tarski asks:
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Tarski’s Problem
FX the free group on X , |X | the cardinal of X .
Around 1945, Tarski asks:
Pb1 (Following Vaught): Is it true that Th(FX ) = Th(FY ), for|X |, |Y | ≥ 2 ?
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Tarski’s Problem
FX the free group on X , |X | the cardinal of X .
Around 1945, Tarski asks:
Pb1 (Following Vaught): Is it true that Th(FX ) = Th(FY ), for|X |, |Y | ≥ 2 ?
Pb2: Is it true that Th(FX ) decidable ?
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Tarski’s Problem
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof.
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.The problem is reduced to show:
FY |= φ(g ; b) ⇒ ∃b′ ∈ FX ,FY |= φ(g ; b′).
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.The problem is reduced to show:
FY |= φ(g ; b) ⇒ ∃b′ ∈ FX ,FY |= φ(g ; b′).FY |= φ(g ; b) ⇒ g ∈ 〈X0〉, b ∈ 〈X0,Y0〉, with X0 ⊆ X ,Y0 ⊆ Yand X0 ∩ Y0 = ∅.
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.The problem is reduced to show:
FY |= φ(g ; b) ⇒ ∃b′ ∈ FX ,FY |= φ(g ; b′).FY |= φ(g ; b) ⇒ g ∈ 〈X0〉, b ∈ 〈X0,Y0〉, with X0 ⊆ X ,Y0 ⊆ Yand X0 ∩ Y0 = ∅.Let X1 ⊆ X \ X0 with |X1| = |Y0|.
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.The problem is reduced to show:
FY |= φ(g ; b) ⇒ ∃b′ ∈ FX ,FY |= φ(g ; b′).FY |= φ(g ; b) ⇒ g ∈ 〈X0〉, b ∈ 〈X0,Y0〉, with X0 ⊆ X ,Y0 ⊆ Yand X0 ∩ Y0 = ∅.Let X1 ⊆ X \ X0 with |X1| = |Y0|.f : Y → Y , f (X \ X1) = X \ X1, f (Y0) = X1, f (X1) = f (Y0), andleaves the rest unchanged, is an automorphism and f (g) = g .
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Tarski’s Problem
Theorem 1 (Vaught, 1955)
If X ⊆ Y , X is infinite, then FX is an elementary subgroup of FY .
Proof. We use Tarski-Vaught test.The problem is reduced to show:
FY |= φ(g ; b) ⇒ ∃b′ ∈ FX ,FY |= φ(g ; b′).FY |= φ(g ; b) ⇒ g ∈ 〈X0〉, b ∈ 〈X0,Y0〉, with X0 ⊆ X ,Y0 ⊆ Yand X0 ∩ Y0 = ∅.Let X1 ⊆ X \ X0 with |X1| = |Y0|.f : Y → Y , f (X \ X1) = X \ X1, f (Y0) = X1, f (X1) = f (Y0), andleaves the rest unchanged, is an automorphism and f (g) = g .Hence FY |= φ(g ; f (b)), f (b) ∈ FX .
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Tarski’s Problem
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Tarski’s Problem
What about free ”object” in other varieties ?
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
Th(Nnc )=Th(Nn′
c′ ) if and only if n = n′ and c = c ′.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
Th(Nnc )=Th(Nn′
c′ ) if and only if n = n′ and c = c ′.
for c ≥ 2, n ≥ 2, Th(Nnc ) is undecidable (Mal’cev, 1960).
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
Th(Nnc )=Th(Nn′
c′ ) if and only if n = n′ and c = c ′.
for c ≥ 2, n ≥ 2, Th(Nnc ) is undecidable (Mal’cev, 1960).
• Scn := the free soluble group of class c and rank n.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
Th(Nnc )=Th(Nn′
c′ ) if and only if n = n′ and c = c ′.
for c ≥ 2, n ≥ 2, Th(Nnc ) is undecidable (Mal’cev, 1960).
• Scn := the free soluble group of class c and rank n.
Th(Scn )=Th(Sc
n′) if and only if n = n′ and c = c ′.
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Tarski’s Problem
What about free ”object” in other varieties ?
• Zn := the free abelian group of rank n.
Th(Zn)=Th(Zm) if and only if n = m.
Th(Zn) is decidable (Szmielew, 1955).
• Ncn := the free nilpotent group of class c and rank n.
Th(Nnc )=Th(Nn′
c′ ) if and only if n = n′ and c = c ′.
for c ≥ 2, n ≥ 2, Th(Nnc ) is undecidable (Mal’cev, 1960).
• Scn := the free soluble group of class c and rank n.
Th(Scn )=Th(Sc
n′) if and only if n = n′ and c = c ′.
for c , n ≥ 2, Th(Scn ) is undecidable (Mal’cev, 1960).
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Tarski’s Problem
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Tarski’s Problem
Remark.
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Tarski’s Problem
Remark. Theorem 1 is still true for free object of varieties.
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Tarski’s Problem
Remark. Theorem 1 is still true for free object of varieties. Inparticular we have:
NcX :=the free nilpotent group of class c , on X
X ⊆ Y ,X infinite ⇒ NcX ≺ Nc
Y ,
ScX :=the free soluble group of class c , on X
X ⊆ Y ,X infinite ⇒ ScX ≺ Sc
Y .
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Tarski’s Problem: Universal theory & Equations
Definition
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Tarski’s Problem: Universal theory & Equations
Definition
• an universal formula φ(z), with free variables z,
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Tarski’s Problem: Universal theory & Equations
Definition
• an universal formula φ(z), with free variables z,
∀xϕ(x , z),
where ϕ(x , z) is a quantifier-free formula.
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Tarski’s Problem: Universal theory & Equations
Definition
• an universal formula φ(z), with free variables z,
∀xϕ(x , z),
where ϕ(x , z) is a quantifier-free formula.
• universal sentence:= universal formula without free variables.
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Tarski’s Problem: Universal theory & Equations
Definition
• an universal formula φ(z), with free variables z,
∀xϕ(x , z),
where ϕ(x , z) is a quantifier-free formula.
• universal sentence:= universal formula without free variables.
• Let G be a group.
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Tarski’s Problem: Universal theory & Equations
Definition
• an universal formula φ(z), with free variables z,
∀xϕ(x , z),
where ϕ(x , z) is a quantifier-free formula.
• universal sentence:= universal formula without free variables.
• Let G be a group. The universal theory of G ,
Th∀(G ):= the set of universal sentences true in G .
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
|X | = ω, FX is embeddable in F2.
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Tarski’s Problem: Universal theory & Equations
|X | = ω, FX is embeddable in F2.Indeed: F2 = 〈a, b|〉, the subgroup 〈a−nban; i ∈ N〉 is isomorphic toFω.
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Tarski’s Problem: Universal theory & Equations
|X | = ω, FX is embeddable in F2.Indeed: F2 = 〈a, b|〉, the subgroup 〈a−nban; i ∈ N〉 is isomorphic toFω.Consequence (combining with Theorem 1):
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Tarski’s Problem: Universal theory & Equations
|X | = ω, FX is embeddable in F2.Indeed: F2 = 〈a, b|〉, the subgroup 〈a−nban; i ∈ N〉 is isomorphic toFω.Consequence (combining with Theorem 1):Th∀(FX ) = Th∀(FY ), for |X |, |Y | ≥ 2.
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Tarski’s Problem: Universal theory & Equations
|X | = ω, FX is embeddable in F2.Indeed: F2 = 〈a, b|〉, the subgroup 〈a−nban; i ∈ N〉 is isomorphic toFω.Consequence (combining with Theorem 1):Th∀(FX ) = Th∀(FY ), for |X |, |Y | ≥ 2.
=⇒ nonabelian free groups have the same universal theory.
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Vaught made the (unpublished) test problem:
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Tarski’s Problem: Universal theory & Equations
Vaught made the (unpublished) test problem:
Is it true that FX |= ∀x∀y∀z(x2y2z2 = 1 ⇒ xy = yx)?
Theorem 2 (Lyndon, 1959)
The answer is yes.
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Tarski’s Problem: Universal theory & Equations
Vaught made the (unpublished) test problem:
Is it true that FX |= ∀x∀y∀z(x2y2z2 = 1 ⇒ xy = yx)?
Theorem 2 (Lyndon, 1959)
The answer is yes.
Consequence: the surface group with presentation
〈x , y , z |x2y2z2 = 1〉
does not satisfy the universal theory of nonabelian free groups.
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Lemma 3 (Mal’cev, 1962)
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Tarski’s Problem: Universal theory & Equations
Lemma 3 (Mal’cev, 1962)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1.
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Tarski’s Problem: Universal theory & Equations
Lemma 3 (Mal’cev, 1962)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y(x2ax2a−1(ybyb−1)−2 = 1 ⇔ x = 1 ∧ y = 1).
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Tarski’s Problem: Universal theory & Equations
Lemma 3 (Mal’cev, 1962)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y(x2ax2a−1(ybyb−1)−2 = 1 ⇔ x = 1 ∧ y = 1).
Consequence: A finite system of equations in a nonabelian freegroup is (effectively) equivalent to a single equation.
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Tarski’s Problem: Universal theory & Equations
Lemma 3 (Mal’cev, 1962)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y(x2ax2a−1(ybyb−1)−2 = 1 ⇔ x = 1 ∧ y = 1).
Consequence: A finite system of equations in a nonabelian freegroup is (effectively) equivalent to a single equation.
Remark. Lemma 3 holds also in nonabelian models of the univeraltheory of nonabelian free groups (Kharlampovich and Myasnikov,1998).
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Lemma 4 (Guervich)
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Tarski’s Problem: Universal theory & Equations
Lemma 4 (Guervich)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1.
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Tarski’s Problem: Universal theory & Equations
Lemma 4 (Guervich)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y([x , ya] = 1∧ [x , yb] = 1∧ [x , yab] = 1 ⇔ x = 1∨y = 1).
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Tarski’s Problem: Universal theory & Equations
Lemma 4 (Guervich)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y([x , ya] = 1∧ [x , yb] = 1∧ [x , yab] = 1 ⇔ x = 1∨y = 1).
Consequence: The disjunction of equations in a nonabelian freegroup is (effectively) equivalent to a finite system of equations.
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Tarski’s Problem: Universal theory & Equations
Lemma 4 (Guervich)
Let F be a free group and a, b ∈ F such that [a, b] 6= 1. Then
F |= ∀x∀y([x , ya] = 1∧ [x , yb] = 1∧ [x , yab] = 1 ⇔ x = 1∨y = 1).
Consequence: The disjunction of equations in a nonabelian freegroup is (effectively) equivalent to a finite system of equations.
Remark. Lemma 4 holds also in nonabelian models of theuniversal theory of nonabelian free groups (Kharlampovich andMyasnikov, 1998).
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Conclusion:Let G be a nonabelian model of the universal theory of nonabelianfree groups.
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Tarski’s Problem: Universal theory & Equations
Conclusion:Let G be a nonabelian model of the universal theory of nonabelianfree groups.
• A quantifier-free formula ϕ(x) is (effectivelly) equivalent to aformula of the form
∨
1≤i≤n
(wi (x , a, b) = 1 ∧ vi (x , a, b) 6= 1)
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Tarski’s Problem: Universal theory & Equations
Conclusion:Let G be a nonabelian model of the universal theory of nonabelianfree groups.
• A quantifier-free formula ϕ(x) is (effectivelly) equivalent to aformula of the form
∨
1≤i≤n
(wi (x , a, b) = 1 ∧ vi (x , a, b) 6= 1)
Definition
A positive formula φ(z), with free variables z , is a formula
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Tarski’s Problem: Universal theory & Equations
Conclusion:Let G be a nonabelian model of the universal theory of nonabelianfree groups.
• A quantifier-free formula ϕ(x) is (effectivelly) equivalent to aformula of the form
∨
1≤i≤n
(wi (x , a, b) = 1 ∧ vi (x , a, b) 6= 1)
Definition
A positive formula φ(z), with free variables z , is a formula
φ(z) := ∀x1∃y1 . . . ∀xn∃yn
∨
1≤i≤n
(∧
1≤j≤pi
wij(x , y , z) = 1)
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Tarski’s Problem: Universal theory & Equations
• A positive formula ϕ(x) is (effectivelly) equivalent to aformula of the form
∀x1∃y1 . . . ∀xn∃yn(w(x , y , z ; a, b) = 1)
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Theorem 5 (Following Kharlampovich&Myasnikov, Merzljakov’sTheorem, 1966)
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Tarski’s Problem: Universal theory & Equations
Theorem 5 (Following Kharlampovich&Myasnikov, Merzljakov’sTheorem, 1966)
Let F be a nonabelian free group.
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Tarski’s Problem: Universal theory & Equations
Theorem 5 (Following Kharlampovich&Myasnikov, Merzljakov’sTheorem, 1966)
Let F be a nonabelian free group. If
F |= ∀x1∃y1 . . . ∀xn∃yn w(x1, y1, . . . , xn, yn; a) = 1,
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Tarski’s Problem: Universal theory & Equations
Theorem 5 (Following Kharlampovich&Myasnikov, Merzljakov’sTheorem, 1966)
Let F be a nonabelian free group. If
F |= ∀x1∃y1 . . . ∀xn∃yn w(x1, y1, . . . , xn, yn; a) = 1,
then, there exist words, with parameters from F ,v1(x1), v2(x1, x2), . . . , vn(x1, x2, . . . , xn), such that
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Tarski’s Problem: Universal theory & Equations
Theorem 5 (Following Kharlampovich&Myasnikov, Merzljakov’sTheorem, 1966)
Let F be a nonabelian free group. If
F |= ∀x1∃y1 . . . ∀xn∃yn w(x1, y1, . . . , xn, yn; a) = 1,
then, there exist words, with parameters from F ,v1(x1), v2(x1, x2), . . . , vn(x1, x2, . . . , xn), such that
F [x1, . . . , xn] |= w(
x1, v1(x1), . . . , xn, v(x1, . . . , xn); a)
= 1.
Here F [x1, . . . , xn] = F ∗ 〈x1, . . . , xn|〉.
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
What about decidability ?
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Tarski’s Problem: Universal theory & Equations
What about decidability ?
Theorem 6 (Makanin)
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Tarski’s Problem: Universal theory & Equations
What about decidability ?
Theorem 6 (Makanin)
(1) There is a an algorithm for recognizing the solvability of anarbitrary equation in a free group (1982).
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Tarski’s Problem: Universal theory & Equations
What about decidability ?
Theorem 6 (Makanin)
(1) There is a an algorithm for recognizing the solvability of anarbitrary equation in a free group (1982).(2) The universal and the positive theory of a nonabelian freegroup is decidable (1985).
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Tarski’s Problem: Universal theory & Equations
What about decidability ?
Theorem 6 (Makanin)
(1) There is a an algorithm for recognizing the solvability of anarbitrary equation in a free group (1982).(2) The universal and the positive theory of a nonabelian freegroup is decidable (1985).
Remark. The decidability of the positive theory follows easillyfrom (1) and Merzljakov’s Theorem.
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
The first step to understand the elementary theory of a free groupis to understand the set of solutions of equations.
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Tarski’s Problem: Universal theory & Equations
The first step to understand the elementary theory of a free groupis to understand the set of solutions of equations.
Theorem 7 (Lorents 1963, Appel 1968, Chiswell &Remeslennikov 2000)
The set of solutions of a system of equations, with one variable, ina free group, is a finite union of cosets of centralizers.
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Tarski’s Problem: Universal theory & Equations
The first step to understand the elementary theory of a free groupis to understand the set of solutions of equations.
Theorem 7 (Lorents 1963, Appel 1968, Chiswell &Remeslennikov 2000)
The set of solutions of a system of equations, with one variable, ina free group, is a finite union of cosets of centralizers.
Remark. Lorents has anounced the theorem without proof and theproof of Appel contains a gap.
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Tarski’s Problem: Universal theory & Equations
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Tarski’s Problem: Universal theory & Equations
Theorem 8
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Tarski’s Problem: Universal theory & Equations
Theorem 8
The set of solutions of a system of equations, with one variable, ina torsion-free 2-free hyperbolic group, is a finite union of cosets ofcentralizers.
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Tarski’s Problem: Equations & Sela’s approach
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters).
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters). LetHw = 〈x |w(x) = 1〉.
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters). LetHw = 〈x |w(x) = 1〉. Let G be a group.
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters). LetHw = 〈x |w(x) = 1〉. Let G be a group.
• If g ∈ G such that G |= w(g) = 1, then there exists ahomomorphism f : Hw → G such that f (x) = g .
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters). LetHw = 〈x |w(x) = 1〉. Let G be a group.
• If g ∈ G such that G |= w(g) = 1, then there exists ahomomorphism f : Hw → G such that f (x) = g .
• Conversely, if f : Hw → G is an homomorphism, then f (x) isa solution of w(x) = 1.
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Tarski’s Problem: Equations & Sela’s approach
Summary of Sela’s approach:Let w(x) = 1 be an equation (to simplify without parameters). LetHw = 〈x |w(x) = 1〉. Let G be a group.
• If g ∈ G such that G |= w(g) = 1, then there exists ahomomorphism f : Hw → G such that f (x) = g .
• Conversely, if f : Hw → G is an homomorphism, then f (x) isa solution of w(x) = 1.
=⇒ correspondance between solutions of w(x) = 1 andHom(Hw ,G ).
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Tarski’s Problem: Equations & Sela’s approach
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.Let F be a free group and fn : H → F be a ”good” sequence ofhomomorphisms.
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.Let F be a free group and fn : H → F be a ”good” sequence ofhomomorphisms.=⇒ we get a sequence of actions of H on the Cayley graph of F .
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.Let F be a free group and fn : H → F be a ”good” sequence ofhomomorphisms.=⇒ we get a sequence of actions of H on the Cayley graph of F .=⇒ one can extracts a subsequence converging in theGromov-Hausdorff topology to an action on a reel tree.
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.Let F be a free group and fn : H → F be a ”good” sequence ofhomomorphisms.=⇒ we get a sequence of actions of H on the Cayley graph of F .=⇒ one can extracts a subsequence converging in theGromov-Hausdorff topology to an action on a reel tree.=⇒ a ”good” action of H on a reel tree.
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Tarski’s Problem: Equations & Sela’s approach
If G has a ”good” action on a ”good” space (like free groups andtorsion-free hyperbolic groups), then one can understand set ofsolutions of equations.Let F be a free group and fn : H → F be a ”good” sequence ofhomomorphisms.=⇒ we get a sequence of actions of H on the Cayley graph of F .=⇒ one can extracts a subsequence converging in theGromov-Hausdorff topology to an action on a reel tree.=⇒ a ”good” action of H on a reel tree.=⇒ a beautiful structure of H(modulo the kernel of the action)and Hom(H,F ).
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Tarski’s Problem: Equations & Sela’s approach
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Tarski’s Problem: Equations & Sela’s approach
Recall that:
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Tarski’s Problem: Equations & Sela’s approach
Recall that:
Definition
Let F be a free group and G a group.
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Tarski’s Problem: Equations & Sela’s approach
Recall that:
Definition
Let F be a free group and G a group.
• A sequence of homomorphisms fn : G → F is convergent if forany g ∈ G , one of the following sets is finite
S1(g) = {n|fn(g) = 1}, S2(g) = {n|fn(g) 6= 1}.
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Tarski’s Problem: Equations & Sela’s approach
Recall that:
Definition
Let F be a free group and G a group.
• A sequence of homomorphisms fn : G → F is convergent if forany g ∈ G , one of the following sets is finite
S1(g) = {n|fn(g) = 1}, S2(g) = {n|fn(g) 6= 1}.
• ker∞(fn) = {g ∈ G |S2(g) is finite}.
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Tarski’s Problem: Equations & Sela’s approach
Recall that:
Definition
Let F be a free group and G a group.
• A sequence of homomorphisms fn : G → F is convergent if forany g ∈ G , one of the following sets is finite
S1(g) = {n|fn(g) = 1}, S2(g) = {n|fn(g) 6= 1}.
• ker∞(fn) = {g ∈ G |S2(g) is finite}.
• H is a limit group, if H = G/ker∞(fn) for some group G anda convergent sequence fn : G → F .
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Tarski’s Problem: Equations & Sela’s approach
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Tarski’s Problem: Equations & Sela’s approach
Link with the universal theory:
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Tarski’s Problem: Equations & Sela’s approach
Link with the universal theory: the following properties areequivalent:
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Tarski’s Problem: Equations & Sela’s approach
Link with the universal theory: the following properties areequivalent:(1) H is a limit group,
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Tarski’s Problem: Equations & Sela’s approach
Link with the universal theory: the following properties areequivalent:(1) H is a limit group,
(2) H is ω-residually free,
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Tarski’s Problem: Equations & Sela’s approach
Link with the universal theory: the following properties areequivalent:(1) H is a limit group,
(2) H is ω-residually free,
(3) H is a model of the universal theory of a free group.
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Tarski’s Problem: Quantifier elimination
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.A formula φ(z) is ∀∃-formula if it is of the form ∀x∃yϕ(x , y , z),where ϕ(x , y , z) is a quantifier-free formula.
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.A formula φ(z) is ∀∃-formula if it is of the form ∀x∃yϕ(x , y , z),where ϕ(x , y , z) is a quantifier-free formula.
Theorem 9 (Sela)
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.A formula φ(z) is ∀∃-formula if it is of the form ∀x∃yϕ(x , y , z),where ϕ(x , y , z) is a quantifier-free formula.
Theorem 9 (Sela)
Let F be a nonabelian free group of finite rank.
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.A formula φ(z) is ∀∃-formula if it is of the form ∀x∃yϕ(x , y , z),where ϕ(x , y , z) is a quantifier-free formula.
Theorem 9 (Sela)
Let F be a nonabelian free group of finite rank. Then Th(F ) hasquantifier elimination down to ∀∃-formulas.
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Tarski’s Problem: Quantifier elimination
Definition
Let T be a theory and S a set of formulas.T has quantifier elimination down to S if for any formula φ(x),there exists a boolean combination of formulas from S , ϕ(x), suchthat T |= ∀x(φ(x) ⇔ ϕ(x)).
T |= φ means M |= T ⇒ M |= φ.A formula φ(z) is ∀∃-formula if it is of the form ∀x∃yϕ(x , y , z),where ϕ(x , y , z) is a quantifier-free formula.
Theorem 9 (Sela)
Let F be a nonabelian free group of finite rank. Then Th(F ) hasquantifier elimination down to ∀∃-formulas.Furtheremore, for any formula φ(x), there exists a booleancombination of ∀∃-formulas, ϕ(x), such that:F |= ∀x(φ(x) ⇔ ϕ(x)), for any free nonabelian group F .
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Tarski’s Problem: the solution
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Tarski’s Problem: the solution
Theorem 10 (Kharlampovich&Myasnikov, Independently Sela)
Fn ≺ Fm for m ≥ n ≥ 2.
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Tarski’s Problem: the solution
Theorem 10 (Kharlampovich&Myasnikov, Independently Sela)
Fn ≺ Fm for m ≥ n ≥ 2.
Theorem 11 (Kharlampovich&Myasnikov)
Th(Fn) is decidable for n ≥ 2.
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Tarski’s Problem: the solution
Theorem 10 (Kharlampovich&Myasnikov, Independently Sela)
Fn ≺ Fm for m ≥ n ≥ 2.
Theorem 11 (Kharlampovich&Myasnikov)
Th(Fn) is decidable for n ≥ 2.
Consequence:FX ≺ FY for X ⊆ Y , |X | ≥ 2.Th(FX ) is decidable.
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More Model Theory: Stability
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More Model Theory: Stability
Definition
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More Model Theory: Stability
Definition
• Let T be a theory and φ(x , y ) a formula.
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More Model Theory: Stability
Definition
• Let T be a theory and φ(x , y ) a formula.φ(x) has the order property,
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More Model Theory: Stability
Definition
• Let T be a theory and φ(x , y ) a formula.φ(x) has the order property, if for any n ∈ N, there exists amodel M |= T and a sequence (ai , bi ), 0 ≤ i ≤ n, in M suchthat:
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More Model Theory: Stability
Definition
• Let T be a theory and φ(x , y ) a formula.φ(x) has the order property, if for any n ∈ N, there exists amodel M |= T and a sequence (ai , bi ), 0 ≤ i ≤ n, in M suchthat:
M |= φ(ai , bj) ⇔ i < j .
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More Model Theory: Stability
Definition
• Let T be a theory and φ(x , y ) a formula.φ(x) has the order property, if for any n ∈ N, there exists amodel M |= T and a sequence (ai , bi ), 0 ≤ i ≤ n, in M suchthat:
M |= φ(ai , bj) ⇔ i < j .
• T is stable, if whenever φ(x , y ) is a formula such thatT ∪ {∃x∃yφ(x , y )} has a model, φ(x , y) is without the orderproperty.
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More Model Theory: Stability
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More Model Theory: Stability
Other notions of ”stability”:
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More Model Theory: Stability
Other notions of ”stability”:
Finite Morley rank ⊆ ω-stability ⊆ superstability ⊆ stability .
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Other notions of ”stability”:
Finite Morley rank ⊆ ω-stability ⊆ superstability ⊆ stability .
Finite Morley rankω-stability
superstability
=⇒ a good abstract notion of dimension.
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
• Abelian-by-finite groups are stable, Z is superstable but notω-stable.
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
• Abelian-by-finite groups are stable, Z is superstable but notω-stable.
• Nonabelian free groups are not superstable (Gibone, Poizat).
![Page 180: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/180.jpg)
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
• Abelian-by-finite groups are stable, Z is superstable but notω-stable.
• Nonabelian free groups are not superstable (Gibone, Poizat).More generally a superstable model of the universal theory ofnonabelian free groups is abelian (Mustafin-Poizat,independently Ould Houcine).
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
• Abelian-by-finite groups are stable, Z is superstable but notω-stable.
• Nonabelian free groups are not superstable (Gibone, Poizat).More generally a superstable model of the universal theory ofnonabelian free groups is abelian (Mustafin-Poizat,independently Ould Houcine).
• Non-cyclic torsion-free hyperbolic groups are not superstable(Ould Houcine).
![Page 182: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/182.jpg)
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Examples:
• Algebraic groups over algerbraically closed fields are of finiteMorley rank, Q has a finite Morley rank.
• Free groups of infinite rank in the variety of nilpotent groupsof class c of exponent pn, p is a prime > c , are ω-stable withinfinite Morley rank (Baudisch).
• Abelian-by-finite groups are stable, Z is superstable but notω-stable.
• Nonabelian free groups are not superstable (Gibone, Poizat).More generally a superstable model of the universal theory ofnonabelian free groups is abelian (Mustafin-Poizat,independently Ould Houcine).
• Non-cyclic torsion-free hyperbolic groups are not superstable(Ould Houcine).
• A group acting nontrivially and without inversions on asimplicial tree is not ω-stable (Ould Houcine).
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Equivalent properties of stability.
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Equivalent properties of stability.
Definition
Let M be a group and A a subset of M.
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Equivalent properties of stability.
Definition
Let M be a group and A a subset of M.
• A sequence (bi : i ∈ I ), is indiscernible over A ,
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Equivalent properties of stability.
Definition
Let M be a group and A a subset of M.
• A sequence (bi : i ∈ I ), is indiscernible over A , if for anyformula with parameters from A, φ(x1, . . . , xn), and i1, . . . , in,j1, . . . , jn, we have
M |= φ(bi1 , . . . , bin) ⇔ M |= φ(bj1 , . . . , bjn).
• A sequence (bi : i ∈ I ), I ordered, is order-indiscernible overA ,
![Page 188: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/188.jpg)
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Equivalent properties of stability.
Definition
Let M be a group and A a subset of M.
• A sequence (bi : i ∈ I ), is indiscernible over A , if for anyformula with parameters from A, φ(x1, . . . , xn), and i1, . . . , in,j1, . . . , jn, we have
M |= φ(bi1 , . . . , bin) ⇔ M |= φ(bj1 , . . . , bjn).
• A sequence (bi : i ∈ I ), I ordered, is order-indiscernible overA , if for any formula with parameters from A, φ(x1, . . . , xn),and i1 < i2 < · · · < in, j1 < · · · < jn, we have
![Page 189: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/189.jpg)
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Equivalent properties of stability.
Definition
Let M be a group and A a subset of M.
• A sequence (bi : i ∈ I ), is indiscernible over A , if for anyformula with parameters from A, φ(x1, . . . , xn), and i1, . . . , in,j1, . . . , jn, we have
M |= φ(bi1 , . . . , bin) ⇔ M |= φ(bj1 , . . . , bjn).
• A sequence (bi : i ∈ I ), I ordered, is order-indiscernible overA , if for any formula with parameters from A, φ(x1, . . . , xn),and i1 < i2 < · · · < in, j1 < · · · < jn, we have
M |= φ(bi1 , . . . , bin) ⇔ M |= φ(bj1 , . . . , bjn).
![Page 190: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/190.jpg)
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Example:
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Example:
Let F be the free group on X = (xi |i ∈ N).
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Example:
Let F be the free group on X = (xi |i ∈ N). Then (xi |i ∈ N) isindiscernible in F (over ∅).
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Example:
Let F be the free group on X = (xi |i ∈ N). Then (xi |i ∈ N) isindiscernible in F (over ∅). Indeed any permutation of X unduce anisomorphism, and isomorphisms preserve satisfaction of formulas.
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The following properties are equivallent:
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The following properties are equivallent:
• T is stable,
![Page 198: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/198.jpg)
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The following properties are equivallent:
• T is stable,
• in any model of T , any order-indiscernible sequence isindiscernible,
![Page 199: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/199.jpg)
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The following properties are equivallent:
• T is stable,
• in any model of T , any order-indiscernible sequence isindiscernible,
• there exists a notion of independence in models of T withgood properties.
![Page 200: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/200.jpg)
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Theorem 12 (Sela)
A trosion-free hyperbolic group is stable.
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Theorem 12 (Sela)
A trosion-free hyperbolic group is stable.
Consequence: existence of a good notion of independence ingroups elementary equivalent to a trosion-free hyperbolic group.
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Theorem 13
Let G be a group. Suppose that G is stable and G ≺ G ∗ Z. ThenG is connected and any positive formula is equivalent to anexistential positive one.
![Page 205: Introduction to Tarski's problem and Model Theory](https://reader030.vdocuments.site/reader030/viewer/2022012614/619c174545dca568d06c1678/html5/thumbnails/205.jpg)
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Theorem 13
Let G be a group. Suppose that G is stable and G ≺ G ∗ Z. ThenG is connected and any positive formula is equivalent to anexistential positive one.
THE END