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Math2306/6406 - Formula sheet Complex Analysis Semester 2, 2022
发布时间:2022-11-05
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Math2306/6406 - Formula sheet
Complex Analysis
Semester 2, 2022
Analytic:
Definition (Analytic). A function f of a complex variable is said to be analytic (or holomorphic, or regular) in an open set S if it has a derivative at every point of S .
If S is not an open set, then we say f is analytic in S if f is analytic in an open set containing S.
Cauchy–Riemann equations:
∂u ∂v ∂u ∂v
∂x ∂y , ∂y ∂x .
A necessary condition for a function f(z) = u(x,y) + iv(x,y) to be differentiable at a point z0 is that the Cauchy–Riemann equations holds at z0 .
Theorem (Sufficient conditions for differentiability). Let f(z) = u(x,y)+iv(x,y) be defined in some open set S containing the point z0 . If the first order partial derivatives of u and v exist in S, are continuous at z0 , and satisfy the Cauchy –Riemann equations at z0 , then f is differentiable at z0 . Moreover,
∂u ∂v
∂x ∂x
∂v ∂u
∂y ∂y
Compact and connected sets: Let f : S → C be continuous. Then, a compact set of S is mapped onto a compact set in f(S), and a connected set of S is mapped onto a connected set of f(S).
Constant functions: If f(z) is analytic in a domain S and f\ (z) = 0 everywhere in S, then f(z) is a constant
in S. If f is analytic in a domain S and |f| is constant there, then f is constant. Continuity of complex functions: If f is differentiable at a point z, then f is continuous at z . Derivative of complex functions: The derivative of f at z0 is given by:
df f(z0 + ∆z) − f(z0 )
dz ∆z→0 ∆z ,
provided the limit exists.
Derivative of a polynomial: For any positive integer n,
zn = nzn −1 .
Entire:
Definition (Entire). We call f analytic at the point z if f is analytic in some neighbourhood of z . If a function f is analytic on the whole complex plane, then it is said to be entire .
Function composition:
Theorem. If limz →z0 g(z) = w0 and limw →w0 f(w) = A, then
z0 f(g(z)) = A = f (z
0 g(z)) .
Harmonic functions: A real-valued function Φ(x,y) is harmonic in a domain S if all of its second-order partial derivatives are continuous in S and it satisfies Φ北北 + Φyy = 0 at each point of S. The functions u(x,y) and v(x,y) are harmonic conjugates of each other if they are harmonic in a domain S and satisfy the Cauchy–Riemann equations at every point of S. If f(z) = u(x,y) + iv(x,y) is analytic in a domain S, then u(x,y) and v(x,y) are harmonic conjugates of each other in S.
Harmonic functions and analytic mappings Let w = u + iv = f(z) = f(x + iy) be an analytic mapping of a domain D in the z-plane onto a domain Ω in the w-plane. If the function Φ(u,v) is harmonic in Ω, then the function ϕ(x,y) = Φ(u(x,y),v(x,y)) is harmonic in D .
L’Hˆopital’s Rule: Suppose f and g are analytic functions at a point z0 and f(z0 ) = g(z0 ) = 0, but g\ (z0 ) 0. Then,
f(z) f\ (z0 )
z →z0 g(z) g\ (z0 ) .
Limit of complex functions: Let f(z) = u(x,y) + iv(x,y), z0 = x0 + iy0 , and w0 = u0 + iv0 . Then, limz →z0 f(z) = w0 if and only if limx→x0 ,y →y0 u(x,y) = u0 and limx→x0 ,y →y0 v(x,y) = v0 .
Properties of continuous functions: If f(z)and g(z) are continuous at z0 , then so are f(z)±g(z), f(z)g(z), and f(z)/g(z) provided g(z0 ) 0.
Properties of differentiable functions: If f and g are differentiable at a point z0 , then
❼ (f ± g)\ (z0 ) = f\ (z0 ) ± g\ (z0 ),
❼ (cf) (z\0 ) = cf (z\0 ) (c ∈ C is a constant),
❼ (fg)\ (z0 ) = f(z0 )g\ (z0 ) + f\ (z0 )g(z0 ),
❼
( ) \ (z0 ) =
if g(z0 ) 0,
❼ and (f ◦ g)\ (z0 ) = f\ (g(z0 ))g\ (z0 ), provided f is differentiable at g(z0 ).