Complex Integration by Substitution
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Theorem
Let $\left[{a \,.\,.\, b}\right]$ be a closed real interval.
Let $\phi: \left[{a \,.\,.\, b}\right] \to \R$ be a real function which has a derivative on $\left[{a \,.\,.\, b}\right]$.
Let $f: A \to \C$ be a continuous complex function, where $A$ is a subset of the image of $\phi$.
If $\phi \left({a}\right) \le \phi \left({b}\right)$, then:
- $\displaystyle \int_{\phi \left({a}\right)}^{\phi \left({b}\right)} f \left({t}\right) \, \mathrm d t = \int_a^b f \left({\phi \left({u}\right)}\right) \phi' \left({u}\right) \, \mathrm d u$
If $\phi \left({a}\right) > \phi \left({b}\right)$, then:
- $\displaystyle \int_{\phi \left({b}\right)}^{\phi \left({a}\right)} f \left({t}\right) \, \mathrm dt = -\int_a^b f \left({\phi \left({u}\right)}\right) \phi' \left({u}\right) ,\ \mathrm d u$
Proof
Let $\operatorname{Re}$ and $\operatorname{Im}$ denote real parts and imaginary parts respectively.
Let $\phi \left({a}\right) \le \phi \left({b}\right)$.
Then:
| \(\displaystyle \int_{\phi \left({a}\right)}^{\phi \left({b}\right)} f \left({t}\right) \, \mathrm d t\) | \(=\) | \(\displaystyle \int_{\phi \left({a}\right)}^{\phi \left({b}\right)} \operatorname{Re} \left({f \left({t}\right) }\right) \, \mathrm d t + i \int_{\phi \left({a}\right)}^{\phi \left({b}\right)} \operatorname{Im} \left({f \left({t}\right) }\right) \, \mathrm d t\) | Definition of Complex Riemann Integral | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \int_a^b \operatorname{Re} \left({ f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u + i \int_a^b \operatorname{Im} \left({ f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u\) | Integration by Substitution | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \int_a^b \operatorname{Re} \left({ f \left({ \phi \left({u}\right) }\right) \phi' \left({u}\right) }\right) \, \mathrm d u + i \int_a^b \operatorname{Im} \left({ f \left({ \phi \left({u}\right) }\right) \phi' \left({u}\right) }\right) \, \mathrm d u\) | Multiplication of Real and Imaginary Parts | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \int_a^b f \left({\phi \left({u}\right)}\right) \phi' \left({u}\right) \, \mathrm d u\) |
Let $\phi \left({a}\right) > \phi \left({b}\right)$.
Then:
| \(\displaystyle \int_{\phi \left({b}\right)}^{\phi \left({a}\right)} f \left({t}\right) \, \mathrm d t\) | \(=\) | \(\displaystyle \int_{\phi \left({b}\right)}^{\phi \left({a}\right)} \operatorname{Re} \left({ f \left({t}\right) }\right) \, \mathrm d t + i \int_{\phi \left({b}\right)}^{\phi \left({a}\right)} \operatorname{Im} \left({ f \left({t}\right) }\right) \, \mathrm d t\) | Definition of Complex Riemann Integral | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \int_b^a \operatorname{Re} \left({f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u + i \int_b^a \operatorname{Im} \left({f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u\) | Integration by Substitution | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle -\int_a^b \operatorname{Re} \left({f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u - i \int_a^b \operatorname{Im} \left({f \left({ \phi \left({u}\right) }\right) }\right) \phi' \left({u}\right) \, \mathrm d u\) | Definition of Definite Integral | ||||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle -\int_a^b f \left({\phi \left({u}\right)}\right) \phi' \left({u}\right) \, \mathrm d u\) |
$\blacksquare$
Sources
- 2001: Christian Berg: Kompleks funktionsteori $\S 2.2$
