From e5f44d97c6652f262c82b5c796c07a7a22a00e90 Mon Sep 17 00:00:00 2001
From: Prefetch
Date: Sun, 4 Jul 2021 20:02:27 +0200
Subject: Expand knowledge base
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+---
+title: "Laplace transform"
+firstLetter: "L"
+publishDate: 2021-07-02
+categories:
+- Mathematics
+- Physics
+
+date: 2021-07-02T15:48:30+02:00
+draft: false
+markup: pandoc
+---
+
+# Laplace transform
+
+The **Laplace transform** is an integral transform
+that losslessly converts a function $f(t)$ of a real variable $t$,
+into a function $\tilde{f}(s)$ of a complex variable $s$,
+where $s$ is sometimes called the **complex frequency**,
+analogously to the [Fourier transform](/know/concept/fourier-transform/).
+The transform is defined as follows:
+
+$$\begin{aligned}
+ \boxed{
+ \tilde{f}(s)
+ \equiv \hat{\mathcal{L}}\{f(t)\}
+ \equiv \int_0^\infty f(t) \exp\!(- s t) \dd{t}
+ }
+\end{aligned}$$
+
+Depending on $f(t)$, this integral may diverge.
+This is solved by restricting the domain of $\tilde{f}(s)$
+to $s$ where $\mathrm{Re}\{s\} > s_0$,
+for an $s_0$ large enough to compensate for the growth of $f(t)$.
+
+
+## Derivatives
+
+The derivative of a transformed function is the transform
+of the original mutliplied by its variable.
+This is especially useful for transforming ODEs with variable coefficients:
+
+$$\begin{aligned}
+ \boxed{
+ \tilde{f}'(s) = - \hat{\mathcal{L}}\{t f(t)\}
+ }
+\end{aligned}$$
+
+This property generalizes nicely to higher-order derivatives of $s$, so:
+
+$$\begin{aligned}
+ \boxed{
+ \dv[n]{\tilde{f}}{s} = (-1)^n \hat{\mathcal{L}}\{t^n f(t)\}
+ }
+\end{aligned}$$
+
+
+
+
+
+
+The exponential $\exp\!(- s t)$ is the only thing that depends on $s$ here:
+
+$$\begin{aligned}
+ \dv[n]{\tilde{f}}{s}
+ &= \dv[n]{s} \int_0^\infty f(t) \exp\!(- s t) \dd{t}
+ \\
+ &= \int_0^\infty (-t)^n f(t) \exp\!(- s t) \dd{t}
+ = (-1)^n \hat{\mathcal{L}}\{t^n f(t)\}
+\end{aligned}$$
+
+
+
+The Laplace transform of a derivative introduces the initial conditions into the result.
+Notice that $f(0)$ is the initial value in the original $t$-domain:
+
+$$\begin{aligned}
+ \boxed{
+ \hat{\mathcal{L}}\{ f'(t) \} = - f(0) + s \tilde{f}(s)
+ }
+\end{aligned}$$
+
+This property generalizes to higher-order derivatives,
+although it gets messy quickly.
+Once again, the initial values of the lower derivatives appear:
+
+$$\begin{aligned}
+ \boxed{
+ \hat{\mathcal{L}} \big\{ f^{(n)}(t) \big\}
+ = - \sum_{j = 0}^{n - 1} s^j f^{(n - 1 - j)}(0) + s^n \tilde{f}(s)
+ }
+\end{aligned}$$
+
+Where $f^{(n)}(t)$ is shorthand for the $n$th derivative of $f(t)$,
+and $f^{(0)}(t) = f(t)$.
+As an example, $\hat{\mathcal{L}}\{f'''(t)\}$ becomes
+$- f''(0) - s f'(0) - s^2 f(0) + s^3 \tilde{f}(s)$.
+
+
+
+
+
+
+We integrate by parts and use the fact that $\lim_{x \to \infty} \exp\!(-x) = 0$:
+
+$$\begin{aligned}
+ \hat{\mathcal{L}} \big\{ f^{(n)}(t) \big\}
+ &= \int_0^\infty f^{(n)}(t) \exp\!(- s t) \dd{t}
+ \\
+ &= \big[ f^{(n - 1)}(t) \exp\!(- s t) \big]_0^\infty + s \int_0^\infty f^{(n-1)}(t) \exp\!(- s t) \dd{t}
+ \\
+ &= - f^{(n - 1)}(0) + s \big[ f^{(n - 2)}(t) \exp\!(- s t) \big]_0^\infty + s^2 \int_0^\infty f^{(n-2)}(t) \exp\!(- s t) \dd{t}
+\end{aligned}$$
+
+And so on.
+By partially integrating $n$ times in total we arrive at the conclusion.
+