tma4115 - calculus 3 · tma4115 - calculus 3 lecture 3, jan 23 toke meier carlsen norwegian...
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TMA4115 - Calculus 3Lecture 3, Jan 23
Toke Meier CarlsenNorwegian University of Science and TechnologySpring 2013
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Review of last week
Last week weintroduced complex numbers, both in a geometric wayand in an algebraic way,defined Re(z), Im(z), |z| and arg(z) for a complexnumber z,defined addition and multiplication of complex numbers,defined complex conjugation,introduced polar representation of complex numbers,computed powers of complex numbers,defined and computed roots of complex numbers.
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Today’s lecture
Today we shalluse complex numbers to solve polynomial equations,look at the fundamental theorem of algebra,introduce the complex exponential function,and study extensions of trigonometric functions to thecomplex numbers.
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Solutions to second degreeequations
If a,b, c are complex numbers and a 6= 0, then the solutions
to the equation az2 + bz + c = 0 are z =−b ±
√b2 − 4ac
2a.
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Problem 1 from the exam fromJune 2012
Solve w2 = (−1 + i√
3)/2. Find all solutions of the equationz4 + z2 + 1 = 0 and draw them in the complex plane. Writethe solutions in the form x + iy .
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Roots of polynomials
If P(z) is a polynomial, then a solution to the equationP(z) = 0 is called a root (or zero) of P(z).z0 is a root of P(z) if and only if (z − z0) is a factor of P(z)(i.e., if P(z) = (z − z0)Q(z) for some polynomial Q(z)).
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The fundamental theorem ofalgebra
Every complex polynomial of degree 1 or higher has a leastone complex root.
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Roots of real polynomials
If w is a root of a real polynomial∑n
k=0 akzk , then w is also aroot of
∑nk=0 akzk .
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Exercise 34 on page xxvii
Check that z1 = 1−√
3i is a zero ofP(z) = z4 − 4z3 + 12z2 − 16z + 16, and find all the zeros ofP.
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Complex functions
A complex function f is a rule that assigns a unique complexnumber f (z) to each number z in some set of complexnumbers (called the domain of f ).
Examples of complex functionsf (z) = Re(z)g(z) = Im(z)h(z) = |z|j(z) = Arg(z)k(z) = zp(z) = z2 − 4z + 6
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Graphic representations ofcomplex functions
We cannot draw the graph of a complex function sincewe would need 4 dimensions to do that.Instead, we can graphically represent the behavior of acomplex function w = f (z) by drawing the z-plane andthe w-plane separately, and showing the image in thew-plane of certain, appropriately chosen set of points inthe z-plane.
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Complex functions
Limits, continuity and differentiability of complex functionscan be defined just as for real functions.
Examples of complex functionsEvery complex polynomial is differentiable, and hencecontinuous.The functions f (z) = Re(z), g(z) = Im(z), h(z) = |z| andk(z) = z are continuous, but not differentiable.The function j(z) = Arg(z) is not continuous.
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The exponential function
One can show that the series∞∑
n=0
zn
n!converges absolutely for
all complex numbers z.
We denote the sum of∞∑
n=0
zn
n!as the exponential function ez .
ez is also the limit limn→∞
(1 +
zn
)n.
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The exponential function and cosand sin
If y is a real number, then
eiy =∞∑
n=0
(iy)n
n!=∞∑
n=0
(iy)2n
(2n)!+∞∑
n=0
(iy)2n+1
(2n + 1)!=
∞∑n=0
(−1)ny2n
(2n)!+ i
∞∑n=0
(−1)ny2n+1
(2n + 1)!= cos(y) + i sin(y).
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The exponential function
One can show that ez1+z2 = ez1ez2. It follows thatez = exeiy = ex(cos y + i sin y) for z = x + iy .
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The exponential function and polarrepresentation
If z 6= 0, then
z = |z|(cos(arg(z)) + i sin(arg(z)))
=eln(|z|)ei arg(z) = eln(|z|)+i arg(z).
The exponential function is not injective (becauseex+iy = ex+i(y+2π)), and does therefore not have an inverse.
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Properties of the exponentialfunction
If z = x + iy , thenez = ez
Re(ez) = ex cos yIm(ez) = ex sin y|ez | = ex
arg(ez) = yOne can also show that ez is differentiable and thatddz ez = ez .
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Sine and cosine
One can show that the series∞∑
n=0
(−1)nz2n
(2n)!and
∞∑n=0
(−1)nz2n+1
(2n + 1)!converge absolutely for all complex numbers
z.
We denote the sum of∞∑
n=0
(−1)nz2n
(2n)!as cos(z), and the sum
of∞∑
n=0
(−1)nz2n+1
(2n + 1)!as sin(z).
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Properties of sin and cos
If z is a complex number, then
cos z =eiz + e−iz
2and sin z =
eiz − e−iz
2i.
sin and cos are periodic with period 2π.sin and cos are differentiable and d
dz sin z = cos z andddz cos z = − sin z.
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Hyperbolic sine and cosine
One can show that the series∞∑
n=0
z2n
(2n)!and
∞∑n=0
z2n+1
(2n + 1)!converge absolutely for all complex numbers z.
We denote the sum of∞∑
n=0
z2n
(2n)!as cosh(z), and the sum of
∞∑n=0
z2n+1
(2n + 1)!as sinh(z).
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Properties of sinh and cosh
If z is a complex number, then
cosh z =ez + e−z
2and sinh z =
ez − e−z
2.
sinh and cosh are periodic with period 2πi .sinh and cosh are differentiable and d
dz sinh z = cosh zand d
dz cosh z = sinh z.
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Plan for tomorrow
Tomorrow we shallstudy second-order linear differential equations,introduce the Wronskian,completely solve second-order homogeneous lineardifferential equations with constant coefficients.
Section 4.1 and 4.3 in “Second-Order Equations” (pagesxxxv-xlv and xlix-lv).
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Second-order linear differentialequations
A second-order linear differential equation is a differentialequation with can be written on the form
y ′′ + p(t)y ′ + q(t)y = g(t).
Such an equation is homogeneous if g(t) = 0.
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