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authorPrefetch2022-10-20 18:25:31 +0200
committerPrefetch2022-10-20 18:25:31 +0200
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tree76b8bfd30f8941d0d85365990bcdbc5d0643cabc /source/know/concept/residue-theorem/index.md
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-rw-r--r--source/know/concept/residue-theorem/index.md27
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diff --git a/source/know/concept/residue-theorem/index.md b/source/know/concept/residue-theorem/index.md
index bcbd6bd..b58e3c2 100644
--- a/source/know/concept/residue-theorem/index.md
+++ b/source/know/concept/residue-theorem/index.md
@@ -8,21 +8,21 @@ categories:
layout: "concept"
---
-A function $f(z)$ is **meromorphic** if it is
+A function $$f(z)$$ is **meromorphic** if it is
[holomorphic](/know/concept/holomorphic-function/)
except in a finite number of **simple poles**,
-which are points $z_p$ where $f(z_p)$ diverges,
-but where the product $(z - z_p) f(z)$ is non-zero
-and still holomorphic close to $z_p$.
-In other words, $f(z)$ can be approximated close to $z_p$:
+which are points $$z_p$$ where $$f(z_p)$$ diverges,
+but where the product $$(z - z_p) f(z)$$ is non-zero
+and still holomorphic close to $$z_p$$.
+In other words, $$f(z)$$ can be approximated close to $$z_p$$:
$$\begin{aligned}
f(z)
\approx \frac{R_p}{z - z_p}
\end{aligned}$$
-Where the **residue** $R_p$ of a simple pole $z_p$ is defined as follows, and
-represents the rate at which $f(z)$ diverges close to $z_p$:
+Where the **residue** $$R_p$$ of a simple pole $$z_p$$ is defined as follows, and
+represents the rate at which $$f(z)$$ diverges close to $$z_p$$:
$$\begin{aligned}
\boxed{
@@ -32,8 +32,8 @@ $$\begin{aligned}
**Cauchy's residue theorem** for meromorphic functions
is a generalization of Cauchy's integral theorem for holomorphic functions,
-and states that the integral on a contour $C$
-purely depends on the simple poles $z_p$ enclosed by $C$:
+and states that the integral on a contour $$C$$
+purely depends on the simple poles $$z_p$$ enclosed by $$C$$:
$$\begin{aligned}
\boxed{
@@ -47,14 +47,14 @@ $$\begin{aligned}
<div class="hidden" markdown="1">
<label for="proof-res-theorem">Proof.</label>
From the definition of a meromorphic function,
-we know that we can decompose $f(z)$ like so,
-where $h(z)$ is holomorphic and $z_p$ are all its poles:
+we know that we can decompose $$f(z)$$ like so,
+where $$h(z)$$ is holomorphic and $$z_p$$ are all its poles:
$$\begin{aligned}
f(z) = h(z) + \sum_{z_p} \frac{R_p}{z - z_p}
\end{aligned}$$
-We integrate this over a contour $C$ which contains all poles, and apply
+We integrate this over a contour $$C$$ which contains all poles, and apply
both Cauchy's integral theorem and Cauchy's integral formula to get:
$$\begin{aligned}
@@ -62,11 +62,12 @@ $$\begin{aligned}
&= \oint_C h(z) \dd{z} + \sum_{p} R_p \oint_C \frac{1}{z - z_p} \dd{z}
= \sum_{p} R_p \: 2 \pi i
\end{aligned}$$
+
</div>
</div>
This theorem might not seem very useful,
-but in fact, by cleverly choosing the contour $C$,
+but in fact, by cleverly choosing the contour $$C$$,
it lets us evaluate many integrals along the real axis,
most notably [Fourier transforms](/know/concept/fourier-transform/).
It can also be used to derive the [Kramers-Kronig relations](/know/concept/kramers-kronig-relations).