From 61b5ce20f25c5785e41574998a12c6d06eb05a5e Mon Sep 17 00:00:00 2001
From: Thomas Albers <thomas@thomaslabs.org>
Date: Wed, 8 Mar 2023 23:43:00 +0100
Subject: Restructure build system and directory structures

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 src/math/spherical-harmonics.org | 119 +++++++++++++++++++++++++++++++++++++++
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+#+title: Solving the laplace equation in Spherical coordinates
+#+author: Thomas Albers Raviola
+#+date: 2022-12-15
+#+setupfile: ../../math_options.org
+
+* Disclaimer
+This site as of now just a technology demonstration and its claims
+should not be taken as true (even though I myself am pretty confident
+they are)
+
+* Laplace equation
+
+{{{beg-eqn}}}
+\Delta \phi = 0
+{{{end-eqn}}}
+
+Solutions to this equation are called harmonics
+
+* Laplace equation in spherical coordinates
+
+Using the definition for the laplace operator in spherical coordinates, it
+follows:
+
+{{{beg-align}}}
+\frac{1}{r^2}\pderiv{}{r}\left(r^2 \pderiv{\phi}{r}\right) + \frac{1}{r^2\sin\theta} \pderiv{}{\theta}\left(\sin\theta\pderiv{\phi}{\theta}\right) + \frac{1}{r^2\sin^2\theta}\frac{\partial^2\phi}{\partial\varphi^2} &= 0\\
+\pderiv{}{r}\left(r^2 \pderiv{\phi}{r}\right) + \frac{1}{\sin\theta} \pderiv{}{\theta}\left(\sin\theta\pderiv{\phi}{\theta}\right) + \frac{1}{\sin^2\theta}\frac{\partial^2\phi}{\partial\varphi^2} &= 0
+{{{end-align}}}
+
+Using sepration of variables $\phi$ becomes
+{{{beg-eqn}}}
+\phi(r, \theta, \varphi) = R(r)Y(\theta, \varphi)
+{{{end-eqn}}}
+
+Applying this to the original equation produces
+
+{{{beg-align}}}
+\pderiv{}{r}\left(r^2 \pderiv{R}{r}\right)Y(\theta, \phi) + \left(\frac{1}{\sin\theta} \pderiv{}{\theta}\left(\sin\theta\pderiv{\phi}{\theta}\right) + \frac{1}{\sin^2\theta}\frac{\partial^2\phi}{\partial\varphi^2}\right)R(r) &= 0\\
+\frac{1}{R(r)}\pderiv{}{r}\left(r^2 \pderiv{R}{r}\right) + \frac{1}{Y(\theta, \phi)}\left(\frac{1}{\sin\theta} \pderiv{}{\theta}\left(\sin\theta\pderiv{\phi}{\theta}\right) + \frac{1}{\sin^2\theta}\frac{\partial^2\phi}{\partial\varphi^2}\right) &= 0
+{{{end-align}}}
+
+Because the terms depend on different independant variables, the only way the
+equation holds is if both terms are constant.
+
+{{{beg-align}}}
+&\frac{1}{R(r)}\pderiv{}{r}\left(r^2 \pderiv{R}{r}\right) = \lambda\\
+&\frac{1}{Y(\theta, \phi)}\left(\frac{1}{\sin\theta} \pderiv{}{\theta}\left(\sin\theta\pderiv{Y}{\theta}\right)+ \frac{1}{\sin^2\theta}\frac{\partial^2Y}{\partial\varphi^2}\right) = -\lambda
+{{{end-align}}}
+
+* Angle dependant term
+We again use separation of variables to solve the partial differential equation
+of the angle dependant term.
+
+{{{beg-eqn}}}
+Y(\theta, \varphi) = \Theta(\theta)\Phi(\varphi)
+{{{end-eqn}}}
+
+Replacing into the equation
+
+{{{beg-eqn}}}
+\lambda\sin^2\theta + \frac{\sin\theta}{\Theta(\theta)}\pderiv{}{\theta}\left(\sin\theta\pderiv{\Theta}{\theta}\right)
++ \frac{1}{\Phi(\varphi)}\frac{\partial^2\Phi}{\partial\varphi^2} = 0
+{{{end-eqn}}}
+
+Based on a similar argument it follows that both terms must be constant, with
+this we may now solve for $\Phi$
+
+{{{beg-eqn}}}
+\frac{1}{\Phi(\varphi)}\frac{\partial^2\Phi}{\partial\varphi^2} = -m^2
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\Phi(\varphi) = e^{-m\varphi{}i}
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\lambda\sin^2\theta + \frac{\sin\theta}{\Theta(\theta)}\pderiv{}{\theta}\left(\sin\theta\pderiv{\Theta}{\theta}\right) = m^2
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+l(l+1)\sin^2(\theta)\Theta(\theta) + \pderiv{}{\theta}\left(\sin\theta\pderiv{\Theta}{\theta}\right) = m^2\Theta(\theta)
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+l(l+1)\sin^2(\theta)\Theta(\theta) + \sin(\theta)\left(\cos(\theta) \Theta'(\theta) + \sin(\theta) \Theta''(\theta)\right) = m^2\Theta(\theta)
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\sin^2(\theta) \Theta''(\theta) + \sin(\theta)\cos(\theta) \Theta'(\theta) + (l(l+1)\sin^2(\theta) - m^2)\Theta(\theta) = 0
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\Theta''(\theta) + \cot(\theta)\Theta'(\theta) + \left(l(l+1) - \frac{m^2}{\sin^2\theta}\right)\Theta(\theta) = 0
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\Theta(\theta) = P(\cos \theta)
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\cot(\theta)\Theta'(\theta) = \cot(\theta) \deriv{}{\theta}P(\cos \theta)=-\cos(\theta) P'(\cos\theta)
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\frac{\text{d}^2}{\text{d}\theta^2}(P(\cos\theta)) = \sin^2(\theta) P''(\cos\theta) - \cos(\theta) P'(\cos\theta)
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+\sin^2(\theta) P''(\cos\theta) - 2\cos(\theta) P'(\cos\theta) + \left(l(l+1)-\frac{m^2}{\sin^2\theta}\right)P(\cos\theta) = 0
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+x = \cos\theta
+{{{end-eqn}}}
+
+{{{beg-eqn}}}
+(1 - x^2)P''(x) - 2xP'(x) + \left(l(l+1)-\frac{m^2}{1-x^2}\right)P(x) = 0
+{{{end-eqn}}}
+
+Solve using Frobenius Method
-- 
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