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D Supplementary Sections

D.1 Reminder from Complex Analysis

The analytic function theory is the most powerful tool in the operator theory. Here we briefly recall few facts of complex analysis used in this course. Use any decent textbook on complex variables for a concise exposition. The only difference with our version that we consider function f(z) of a complex variable z taking value in an arbitrary normed space V over the field ℂ. By the direct inspection we could check that all standard proofs of the listed results work as well in this more general case.

Definition 1 A function f(z) of a complex variable z taking value in a normed vector space V is called differentiable at a point z0 if the following limit (called derivative of f(z) at z0) exists:
f′(z0)=
 
lim
Δ z→ 0
f(z0z)−f(z0)
Δ z
. (108)
Definition 2 A function f(z) is called holomorphic (or analytic) in an open set Ω⊂ℂ it is differentiable at any point of Ω.
Theorem 3 (Laurent Series) Let a function f(z) be analytical in the annulus r<z<R for some real r<R, then it could be uniquely represented by the Laurent series:
f(z)=
k=−∞
ckzk,     for some ck∈ V. (109)
Theorem 4 (Cauchy–Hadamard) The radii r and R, (r′<R) of convergence of the Laurent series (109) are given by
r′= 
 
liminf
n→ ∞
⎪⎪
⎪⎪
cn⎪⎪
⎪⎪
1/n   and   
1
R
=
 
limsup
n→ ∞
⎪⎪
⎪⎪
cn⎪⎪
⎪⎪
1/n. (110)

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