jean-guillaume eon - ufrj
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ECM-23, 4-6 August 2006, Leuven. Graph theory: fundamentals and applications to crystallographic and crystallochemical problems . The vector method. Jean-Guillaume Eon - UFRJ. Summary. Motivation Fundamentals of graph theory Crystallographic nets and their quotient graphs - PowerPoint PPT PresentationTRANSCRIPT
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Graph theory: fundamentals and applications to crystallographic and
crystallochemical problems.The vector method
Jean-Guillaume Eon - UFRJ
ECM-23, 4-6 August 2006, Leuven
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Summary
• Motivation• Fundamentals of graph theory• Crystallographic nets and their quotient graphs• Space group and isomorphism class• Topological and geometric properties
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Objectives
• Topology and Geometry: Description and Prevision of Crystal Structures
• Topological properties: Crystallographic Nets and Quotient Graphs
• Geometric properties: Crystal Structures as Embeddings of Crystallographic Nets
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Crystal structures as topological objects
Crystal 1-Complex Net
Chemicalobject
Geometric object
Topological object
Atom Point Vertex
Bond Line Edge
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Example: ReO3
1-Complex Crystallographic net= Embedding of the
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Graph theory (Harary 1972)
A graph G(V, E, m) is defined by:– a set V of vertices,– a set E of edges,– an incidence function m from E to V2
u
v
w
m (e1) = (u, v)
m (e2) = (v, w)
m (e3) = (w, u)
V = {u, v, w} E = {e1, e2, e3}
e1e2
e3
e1 = uve1
-1 = vu
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Simple graphs and multigraphs
loopMultiple edges
Simple graph: graph without loops or multiple edges
Multigraph: graph with multiple edges
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Some elementary definitions
• Order of G: |G| = number of vertices of G• Size of G: ||G|| = number of edges of G• Adjacency: binary relation between edges or vertices• Incidence: binary relation between vertex and edge• Degree of a vertex u: d(u) = number of incident edges
to u (loops are counted twice)• Regular graph of degree r: d(u) = r, for all u in V
Adjacentvertices
Adjacentedges
Incidence relationship
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Walks, paths and cycles• Walk: alternate sequence of incident vertices
and edges
• Closed walk: the last vertex is equal to the first = the last edge is adjacent to the first one
• Trail: a walk which traverses only once each edge
• Cycle: a closed path
• Forest: a graph without cycle
• Path: a walk which traverses only once each vertex
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a.b.j.i trail
a.b.j.d path
b.j.d.e.f.g.h.i closed trail b.j.i cycle
Edges only are enough to define the walk!
b.j.i.b.c walk
b.j.i.b.j.i closed walk
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Nomenclature
• Pn path of n edges• Cn cycle of n edges• Bn bouquet of n loops• Kn complete graph of n vertices• Kn
(m) complete multigraph of n vertices with all edges of multiplicity m
• Kn1, n2, ... nr complete r-partite graph with r sets of ni vertices (i=1,..r)
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C5
P3
B4
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K3(2)K4
K2, 2, 2
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Connectivity
• Connected graph: any two vertices can be linked by a walk
• Point connectivity: κ(G), minimum number of vertices that must be withdrawn to get a disconnected graph
• Line connectivity: λ(G), minimum number of edges that must be withdrawn to get a disconnected graph
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Determine κ(G) and λ(G)
G
λ(G) = 3
κ(G) = 2
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Subgraph of a graphComponent = maximum connected subgraphTree = component of a forest
T1 T2
G = T1 T2
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Spanning graph = subgraph with the same vertex set as the main graph
G
T: spanning tree of G
T
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Hamiltonian graph: graph with a spanning cycle
Show that K2, 2, 2 is a
hamiltonian graph
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Eulerian graph: graph with a closed trail “covering” all edges
Show that K2, 2, 2 is an Eulerian graph
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Distances in a graph
•Length of a walk: ||W|| = number of edges of W
•Distance between two vertices A and B: d(A, B) length of a shortest path (= a geodesic)
A
B A---B|=3|A---B|=4d(A, B)=3
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Morphisms of graphs
• A morphism between two graphs G(V, E, m) and G’(V’, E’ m’) is a pair of maps fV and fE between the vertex and edge sets that respect the adjacency relationships:
for m (e) = (u, v) : m’ {f E (e)} = ( f V (u), f V (v))
or: f(uv) = f(u)f(v)
• Isomorphism of graphs: 1-1 and onto morphism
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e3
e1
e2
v
w
C3
f
u
b
a
e4
e5
f(u) = f(v) = a f(w) = b
f(e1) = e4 f(e2) = f(e3) = e5
Example: an onto morphism in non-oriented graphs
f(vw) = f(v)f(w)
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Aut(G): group of automorphisms
• Automorphism: isomorphism of a graph on itself.
• Aut(G): group of automorphisms by the usual law of composition, noted as a permutation of the (oriented) edges.
v
u
w
e1e2
e3
C3Generators for Aut(C3) order 6
(e1, e2, e3) order 3
(e1, e3-1) (e2, e2
-1) order 2{
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Quotient graphs
• Let G(V, E, m) be a graph and R < Aut(G)• V/R = orbits [u]R of V by R• E/R = orbits [uv]R of E by R• G/R = G(V/R, E/R, m*) quotient graph, with
m* ([uv]R) = ([u]R, [v]R)
• The quotient map: qR(x) = [x]R (for x in V or E), defines a graph homomorphism.
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e3
e1
e2
u v
w
R: generated by (e1, e2, e3)( noted R = <(e1, e2, e3)> )
C3
[u]R
[e1]R
B1 = C3/R
qR
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u v
wx
e1
e2
e3
e4
R = <(u, v)(x, w)>
Find the quotient graph C4/R
C4[u]R
[x]R[e2]R
[e3]R
[e1]R
qR
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Cycle and cocycle spaces on Z
• 0-chains: L0={∑λiui for ui V and λi in Z}• 1-chains: L1={∑λiei for ei E and λi in Z}• Boundary operator: ∂e = v – u for e = uv• Coboundary operator: δu = ∑ei for all ei = uvi
• Cycle space: C = Ker(∂) (cycle-vector e : ∂e = 0)• Cocycle space: C* = Im(δ) (cocycle-vector c = δu)• dim(cycle space): = ||G|| - |G| +1 (cyclomatic
number)
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Cycle and cocycle vectors
A
B
C
D
1 2
3
4
56
Cycle-vector: e1+e2-e3
Cocycle-vector: δA = -e1+e2+e4
∂e1 = A - D
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Cycle and cut spaces on F2 = {0, 1}(non oriented graphs)
• The cycle space is generated by the cycles of G• Cut vector of G = edge set separating G in
disconnected subgraphs• The cut space is generated by δu (u in V)
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u
v
w
Find δ(u + v + w)
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u
v
wδu
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u
v
w
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u
v
w δu + δv
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u
v
w δu + δv
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u
v
w
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u
v
w δu + δv + δw
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u
v
w
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Complementarity: L1 = C C*
• Scalar product in L1: ei.ej = ij
• dim(C*) = |G|-1
• dim(C) + dim(C*) = ||G||
• C C*
uC6
C6 . u = 0
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Example: K2(3)
A B
e1
e2
e3
L1: e1, e2, e3 Natural basis (E)
C: e2 – e1, e3 – e2
C*: e1 + e2 + e3} Cycle-cocycle basis (CC)
CC = M.E M =-1 1 0 0 -1 1 1 1 1
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Write the cycle-cocycle matrix for the graph below
A
B C
e1
e2 e3
e4
e5
M =
1 -1 0 0 0 0 1 -1 0 1 0 0 1 -1 0 1 1 1 1 0-1 -1 0 0 1
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Periodic graphs
• (G, T) is a periodic graph if:
• G is simple and connected,
• T < Aut(G), is free abelian of rank n,
• T acts freely on G (no fixed point or edge),
• The number of (vertex and edge) orbits in G by T is finite.
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Example: the square net
V = Z2
E = {pq | q-p = ± a, ± b}
a = (1,0), b = (0,1)
T = {tr: tr (p) = p+r, r in Z2}
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Invariants in perodic graphs:1. rings
• Topological invariants are conserved by graph automorphisms
• A ring is a cycle that is not the sum of two smaller cycles
• A strong ring is a cycle that is not the sum of (an arbitrary number of) smaller cycles
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Example 1: the square net
A cycle (with shortcuts)
Sum of three strong rings
A strong ring
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Example 2: the cubic net
Strong rings:
Cz = ABCDA Cx = BEFCB Cy = CFGDC
A
B C
D
E F
G
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A cycle that is not a ring: CBEFGDC
A
B C
D
E F
G
CF = shortcut
= Cx + Cy
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A ring that is not a strong ring: ABEFGDA
No shortcut!
A
B
D
E F
G
C
= Cx + Cy + Cz
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Invariants in periodic graphs: 2. Infinite geodesic paths
• Given a graph G, a geodesic L is an infinite, connected subgraph of degree 2, for which, given two arbitrary vertices A and B of L, the path AB in L is a geodesic path between A and B in G.
• L is a strong geodesic if the path AB in L is the unique geodesic between A and B in G.
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Example 1: the square net
Strong geodesics
Geodesics
Yi
Xj
Infinite path (shortcut)
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Are the following infinite paths geodesicsor strong geodesics?
Strong geodesic
Example 2: the β-W net
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Strong geodesic
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Geodesic
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Example 3: the 34.6 net
Not every periodic graph has strong geodesics!
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Geodesic fiber
• 1-periodic subgraph F of a periodic graph G, such that:
• for any pair of vertices x and y, F contains all geodesic lines xy of G, and
• F is minimal, in the sense that no subgraph satisfies the above conditions.
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Example: the β-W net
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The 34.6 net
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Crystallographic nets
• Crystallographic net: simple 3-connected graph whose automorphism group is isomorphic to a crystallographic space group.
• n-Dimensional crystallographic space group: a group Γ with a free abelian group T of rank n which is normal in Γ, has finite factor group Γ/T and whose centralizer coincides with T.
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Local automorphisms
• Automorphism f with an upper bound b for the distance between a vertex and its image:
d{u, f (u)} < b for all u• The set L(N) of local automorphisms of an infinite
graph N forms a normal subgroup of Aut(N)• A net is a crystallographic net iff the group of
local automorphisms L(N) is free abelian and admits a finite number of (vertex and edge) orbits
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Automorphisms and geodesics
• Graph automorphisms map (strong) geodesics on (strong) geodesics.
• A strong geodesic is said to be along f if it is invariant by the local automorphism f.
• Two strong geodesics are parallel if they are invariant by the same local automorphism.
• Local automorphisms map strong geodesics on parallel ones.
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(0,0)
Example: the square net
(i, j)
Conversely: Let f be any local automorphism with f(0,0) = (i,j); then, we will see that ti, j
-1. f = 1
T={ti,j: ti,j (m, n) = (m+i, n+j)} : T L(N)
Y0 Yi
X0
Xj
Y0 invariant by t0,1 as Yi
X0 invariant by t1,0 as Xj
fixes X0 and Y0
fixes the whole net
L(N) = T
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Example: the square net
Simple,
Every automorphism of the infinite graph can beassociated to an isometry of the plane
Crystallographic net
4-connected
L(N) = T free abelian
One vertex orbit
Two edge orbits
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Quotient graph of a periodic graph: the square net
Q = G/T, T translation subgroup T of the automorphism group
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Example: the β-W net
Show that β-W is a crystallographic net andFind its quotient graph
at
bt ct
t in Z2, i=(1,0), j=(0,1)
V = {at , bt , ct}
E = {atbt , atct , btct , atbt+j , atct+j , ctbt+i}
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β-W net: quotient graph
a
b c
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ReO3: vertex and edge lattices
O 1 O 1O 2
O 2
O 3
O 3
R e e1e2
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Quotient graph: ReO3
O 1
e 2 e 1
O 2O 3
e 3
e 4 e 5
e 6R e
K1,3(2) K1,3
(2)
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Structure Quotient graph
NaCl K2(6)
CsCl K2(8)
ZnS (sphalerite) K2(4)
SiO2 (quartz) K3(2)
CaF2 (fluorite) K1,2(4)
Simple structure types
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Symmetry
: Space group of the crystal structure
• T: Normal subgroup of translations/T: Factor group
• Aut(G): Group of automorphism of the quotient graph G
/T is isomorphic to a subgroup of Aut(G)
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ReO3: /T Aut(G) m3m
O 1 O 1O 2
O 2
O 3
O 3
R e e1e2
• C4 Rotation: (e1, e3, e2, e4)• C3 Rotation: (e1, e3, e5) (e2, e4, e6)
• Mirror: (e1, e2)
O 1
e 2 e 1
O 2O 3
e 3
e 4 e 5
e 6R e
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Drawbacks of the quotient graph
• Non-isomorphic nets can have isomorphic quotient graphs
• Different 1-complexes with isomorphic nets can have non-isomorphic quotient graphs
• Different 1-complexes can have both isomorphic nets and isomorphic quotient graph!
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A
BC
D
A AB B
C
C
D
D
Different nets with isomorphic quotient graphs
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A
BC
D
AAB
B
C
C
D
D
A
D
BC
A (0 1 )
B (1 0 )
0 1
1 0
b
a
b
a
1 00 1
Vector method: the labeled quotient graph as a voltage graph
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β-W net: Find the labeled quotient graph
at
bt ct
t in Z2, i=(1,0), j=(0,1)V = {at , bt , ct}E = {atbt , atct , btct , atbt+j , atct+j , ctbt+i}
A
B C10
01 01
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The minimal net
• Translation subgroup isomorphic to the cycle space of the quotient graph G (dimension equal to the cyclomatic number)
• Factor group: isomorphic to Aut(G)• Unique embedding of maximum symmetry:
the archetype (barycentric embedding for stable nets)
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Example 1: graphite layer - K2(3)
1
2
3
A B
1-chain space: 3-dimensional
Two independent cycles: a = e1-e2, b = e2-e3
One independent cocycle vector: e1+ e2+e3
e1
e2
e3
ab
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1
2
3
A B
Generators of Aut[K2(3)]:
• (e1, e2, e3)• (ei , -ei)(A, B)• (e1, -e3)(e2, -e2)(A, B)
Linear operations in the cycle space (point symmetry):
(a, b) = (e1 – e2, e2 – e3) → (e2 – e3, e3 – e1) = (b, - a - b)
In matrix form: 0 -1
1 -1γ{(e1, e2, e3)} =
0 1
1 0γ{(e1, -e3)( e2, -e2)} =
-1 0
0 -1γ{(ei, -ei)} =
p6mm
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Embedding in E3
Inversion of the cycle-cocycle Matrix
a = e1 – e2
b = e2 – e3
c = e1 + e2 + e3
a 1 -1 0 e1
b = 0 1 -1 x e2
c 1 1 1 e3
e1 2 1 1 ae2 = 1/3 -1 1 1 x be3 -1 -2 1 c
Barycentric embedding: c = 0
e1 = 1/3 (2a + b)e2 = 1/3 (-a + b)e3 = 1/3 (-a – 2b)
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Atomic positions
p6mmGenerator 0 -1 -1 0 0 1 1 -1 0 -1 1 0Translation (0 0) (0 0) (0 0)
A: x + TB: x + e1 + T
After inversion: A → -x + T = B = x + 2a/3 + b/3 + T-2x = 2a/3 + b/3 + TWith t = b , x = -a/3 -2b/3 = 2a/3 + b/3
A: 2a/3 + b/3 + TB: a/3 + 2b/3 + T
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Example 2: hyper-quartz
1
2
34
5
6
Archetype: 4-dimensionalTetrahedral coordenationCubic-orthogonal family:(e1-e4), (e2-e5), (e3-e6), ∑ei
Lengths: √2, √2, √2, √6Space group: 25/08/03/003
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Nets as quotients of the minimal net
100
010 001
<111> = translation subgroup generated by 111 N/<111>: T = <{100, 010}>
N (T = Z3) N/<111>-1 -1 0
ab
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(Periodic) voltage net
ab
100010
= 1 1 1
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Embeddings as projections of the archetype
A
B
C
D
1 2
3
4
56
3-d archetype,2-d structures: 1-d kernel
3-cycle: 3.92, 93 4-cycle: 4.82
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A
BC
D0 1
1 0
A
BC
D0 0 1
0 1 0
1 0 0
A
D
BC
1 00 1
A
D
BC
0 1 00 0 1
1 0 0
S r[S i ]
( 4 3 2 )2
I1
(4 .8 )4
2
p m m(3 .9 ,9 )
3 1
2 3
p m
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Check the space group of 4.82
A
B
1 2
3
4
56
DC
Kernel: c = e4 + e6 + e3 – e2
Generators:(e4,e6,e3,-e2)(e1,-e5,-e1,e5)(A,B,C,D)(e4,-e6)(e2,e3)(e1,-e1)(A,C)
Cycle basis:a = e1 + e4 + e5 – e3
b = e1 + e2 - e5 + e6
Notice: a ┴ b ┴ c
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Point symmetry
(a, b) → (-e5 + e6 + e1 + e2, -e5 – e4 – e1 + e3) = (b, -a)
(a, b) → (-e1 – e6 + e5 – e2, -e1 + e3 - e5 – e4) = (-b, -a)
p4mmGenerator 0 -1 0 -1 1 0 -1 0 Translation (0 0) (0 0)
Walk: A → B → C → D → AFinal translation : c = 0
e4 e6 e3 -e2
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Find the space group of quartz
1
2
34
5
6
a = 1000 = e1 - e4
b = 0100 = e2 - e5
c = 0010 = e3 - e6
d = 0001 = e4 + e5 + e6
Hyper quartz = N[K3(2)]
Quartz = N[K3(2)]/<1110>
A
B C
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Point symmetry
Generators: (1,4)(2,5)(3,6) order 2 (1,2,3)(4,5,6)(A,B,C) order 3 (1,6)(2,5)(3,4)(B,C) order 2
(e1 + e2 + e3 - e4 - e5 - e6 = 0 )
Basis: (a, b, d)
order 12
: (a, b, d) (-a, -b, d) C2 rotation d : (a, b, d) (b, -a-b, d) C3 rotation d: (a, b, d) (-a-b, b, -d) C2 rotation d
C6 rotation
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Translation part
Associated to the C3 rotation:
Walk: A → B → C → A
Final translation : e1+e2+e3 = d 31 or 62
e1 e2 e3
For and , t=0 (fixed points) P6222
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Isomorphism class of a net
ab
Find the labeled quotient graph for the double cell
a bb
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Isomorphism class of a net
4, a
bb
1 3 2
Automorphisms of the quotient graph G, which leave the quotient group of the cycle space by its kernel invariant, can be used to extend the translation group if they act freely:
t(o.q-1o [W]) = o.q-1
o [.(W)] ,where δ is a path in G from O to (O)
Kernel: e1-e2
: (e1, e2)(e3, e4)
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Isomorphism class of a nett(o. q-1
o [W]) = o.q-1o [.(W)]
t: new translationo: arbitrary origin in N: boundary operatorq: N G = N/TW: walk in G starting at O=q(o)q-1
o [W]: walk in N starting at o and projecting on Wo. q-1
o [W]: defines a vertex of Nδ: a fixed, arbitrary path in G from O to (O): (extension) automorphism of the quotient graph G
W = OO (path nul): t(o) = o.q-1o
[]
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Isomorphism class of a net
4, a
bb
1 3 2
Kernel: e1-e2
Extension by = (e1, e2)(e3, e4)
b
A B
{A, B}
{e1, e2} {e3, e4}
With o in q-1(A) and = e3 : t (o) = o.q-1o [e3]
t 2 (o) = t (o.q-1o [e3]) = o.q-1
o [e3.(e3)] = o.q-1o [e3.e4] = a(o)
t2 = at ?
ab
ab
t:
t(o. q-1o [W]) = o.q-1
o [.(W)]
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Show that cristobalite has the diamond net
100
101
001
010010
e1
e8e7
e6e5e4e3
e2
A
C
B
D
Kernel: e2 – e1 = e7 – e8
e3 – e4 = e6 – e5
(1,8)(2,7)(3,6)(4,5)(A,D)(B,C)
{A,D} {B,C}
{1,8}
{2,7}, 100
{4,5}, k
e8 – e4 + e1 – e5 = 001 → e1 – e5 + e8 – e4 = 001
2k = e4 – e8 + e5 – e1 = 0 0 -1
{3,6}-{4,5} = 010 l = 010 + k
{3,6}, l
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Non-crystallographic nets
Local automorphisms = T x {I, σ} : abelian but not free!
T
σ
T = <t: t (xm) = xm+1 , x = a, b>
a
b
t te
A Bσ: (e, -e)
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Non-crystallographic nets
Local automorphisms = T’ x {I, σ}
T’
σ
T’ = <t’: t’ (am) = bm+1 , t (bm) = am+1 >
a
b
t’
-t’
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tfc and derived nets
A
B C001
100 010
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A
B C
10 01
tfc/<(001)>
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A
B C
1 1
tfc / < (001), (110) >
Local automorphisms = not abelianBut : (b0, c0) ↔ (b, c)
b0
c0
T
The 1-periodic graph is a geodesic fiber
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Projection of a geodesic fiber on the quotient graph: the β-W net
at
bt ct
A
B C10
01 01
F
G G/T
F/<01> is asubgraph of G/T
01 01
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The 34.6 net (G/T = K6)
10
10
10
11
(Counterclockwiseoriented cycles)
21
G 21
G 10
10-1 0
-1 010
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Determine the quotient graph G/T of the 32.4.3.4 net,
list all the cycles of G/T; find the strong geodesic lines
of 32.4.3.4 and its geodesic fibers.
a
d
b
c
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A
D
B
C
10
10
10 01
0101
2-cyclesAB, CD: 10AD, BC: 01
3-cyclesABC: 00, 01, 11, 10ABD: 00, 01, -1 0, -1 1ACD: 00, -1 0, 0 -1, - 1 -1BCD: 00, 0 -1, 1 -1, 10
4-cyclesABCD: 00, 01, 10, 11, 1 -1ABDC: 01, 11, -1 1ADBC: 1 -1, 10, 11
Strong geodesic lines: 10, 01
disjoint cycles
tangent cycles
Geodesic fibers: 11, -1 1
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a
d
b
c
A
D
B
C
10
10
G 10
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A
D
B
C
10 01
G 11
01
a
d
b
c01
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A
D
BC
11
G 11
a
d
b
c
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Geodesic fibers and local automorphisms
• Local automorphisms send geodesic fibers to parallel geodesic fibers.
• The absence, in 2-connected labeled quotient graphs, of automorphism that leave unchanged the vector labels of any loop or cycle ensures the derived graph is a crystallographic net.
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Cycle figure
10
0 -1
10
01 11
-1 0
0 -1-1 -1
Def: vector star in the direction of geodesic fibers
Graphite layer
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Topological density
ρ = lim ∑k=1,r Ck / rn
Ck: kth coordination numbern : dimension of the net
r→∞
ρ = Z.{Σσ f(σ)}/ n! Z : order of the quotient graphf(σ) : frequency of the face σ of the cycles figure= inverse product of the lengths of the verticesof the face of the cycles figure
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Topological density: graphite layer
10
01 11
-1 0
0 -1-1 -1
σ1
f(σ1) = 1/(2.2)
ρ = Z.{Σσ f(σ)}/ n!
ρ = 2.6.(2.2)-1/2! = 3/2
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Topological density: net 32.4.3.4
10-1 0
01
0 -1
11-1 1
-1 -1 1 -1
3-cycle
2-cycle
ρ = 4.8.(2.3)-1/2! = 8/3
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Find the topological density the β-W net
B C
01 01
A
1010
01
0 -1
11-1 1
-1 -1 1 -1
3-cycle
2-cycle
ρ = 3.8.(2.3)-1/2! = 2
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Plane nets
Embedding of the quotient graph in the torus
A
B C
10 01
a
a
bb
Face boundaries project on oriented trails ofthe quotient graph
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Oriented trails are based on oriented edges
e1, 10 e2, 01
A
B C
a b10
c
a0-1 b1-1a1-1
c10
e1, 10 e2, 01
A
B C
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K4 revisited
3.92, 93 4.82
Two classes of trails: 3-cycles and 4-cycles
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A plane net from K3, 3
Cyclomatic number:υ = 9 – 6 + 1 = 4
Kernel : two tangent 4-cycles
a
a
b b
4.102, 42.10
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Topological invariants of nets in projection on the quotient graph
• Strong geodesic• Geodesic fiber
• Strong ring
• Shorter isolated cycle• Connected subgraph of
shorter cycles (of equal length) with equal or opposite net voltages
• Set of edge-disjoint cycles with nul net voltage
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Non-ambiently isotopic embeddings of the same net
ThSi2 SrSi2
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Self-interpenetrated ThSi2
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Self-interpenetrated SrSi2
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Some references• Beukemann A., Klee W.E., Z. Krist. 201, 37-51 (1992)• Carlucci L., Ciani G., Proserpio D.M., Coord. Chem. Rev. 246, 247-289 (2003)• Chung S.J., Hahn Th., Klee W.E., Acta Cryst. A40, 42-50 (1984)• Delgado-Friedrichs O., Lecture Notes Comp. Sci. 2912, 178-189 (2004)• Delgado-Friedrichs O., O´Keeffe M., Acta Cryst. A59, 351-360 (2003)• Blatov V.A., Acta. Cryst. A56, 178-188 (2000)• Eon J.-G., J. Solid State Chem. 138, 55-65 (1998)• Eon J.-G., J. Solid State Chem. 147, 429-437 (1999)• Eon J.-G., Acta Cryst. A58, 47-53 (2002)• Eon J.-G., Acta Cryst, A60, 7-18 (2004)• Eon J.-G., Acta Cryst A61, 501-511 (2005)• Harary F., Graph Theory, Addison-Wesley (1972)• Klee W.E., Cryst. Res. Technol., 39(11), 959-960 (2004)• Klein H.-J., Mathematical Modeling and Scientific Computing, 6, 325-330 (1996)• Schwarzenberger R.L.E., N-dimensional crystallography, Pitman, London (1980)• Gross J.L, Tucker T.W., Topological Graph Theory, Dover (2001)