adds 1.14
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tex/1.14.tex
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tex/1.14.tex
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\documentclass{article}
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% set up telugu
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\usepackage{fontspec}
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\newfontfamily\telugufont{Potti Sreeramulu}[Script = Telugu]
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\usepackage{polyglossia}
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\setdefaultlanguage{english}
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\setotherlanguage{telugu}
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%other packages
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\usepackage{amsmath}
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\usepackage{amssymb}
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\usepackage{physics}
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\usepackage{siunitx}
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\usepackage{todonotes}
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\usepackage{luacode}
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\usepackage{titling}
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\usepackage{enumitem}
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% custom deepak packages
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\usepackage{luatrivially}
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\usepackage{subtitling}
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\usepackage{cleveref}
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\begin{luacode*}
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math.randomseed(31415926)
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\end{luacode*}
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\newcommand*\mean[1]{\overline{#1}}
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\title{Problem 1.14}
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\subtitle{Width of the Height Distribution}
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\author{\begin{telugu}హృదయ్ దీపక్ మల్లుభొట్ల\end{telugu}}
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% want empty date
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\predate{}
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\date{}
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\postdate{}
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% !TeX spellcheck = en_GB
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\begin{document}
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\maketitle
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Given a Gaussian distribution
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\begin{equation}
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\mathcal{N}(x | \mu, \sigma^2) = \frac{1}{\sqrt{2 \pi \sigma^2}} e^{\flatfrac{-\left(x - \mu\right)^2}{\left(2 \sigma^2\right)}}
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\end{equation}
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Given a sample of $N$ heights $x_n$, the likelihood that they were drawn from a particular normal distribution with $\mu$ and $\sigma^2$ is
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\begin{equation}
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P(\left[x_n \right] | \mu, \sigma^2) = \prod_{n = 1}^N \mathcal{N}(x_n | \mu, \sigma^2).
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\end{equation}
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This is a nightmare.
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Can we fix it to be easier to deal with?
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\begin{enumerate}[label=(\alph*)]
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\item Write $P(\left[x_n \right] | \mu, \sigma^2)$ as a formula depending only on $N$, the sample mean $\mean{x}$ and $S = \sum_n \left(x_n - \mean{x} \right)^2$.
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\item Show that $P$ takes its maximum at $\mu_{ML} = \mean{x}$ and $\sigma_{ML} = \sqrt{\flatfrac{S}{N}}$.
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\item Assume entire population is drawn from (not necessarily Gaussian) distribution with variance $\left<x^2\right>_{samp} = \sigma_0^2$.
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Let $\mu = 0$ for the population.
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Show that $\left< \sigma_{ML}^2 \right>_{samp} = \frac{N - 1}{N} \sigma_0^2$
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\end{enumerate}
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Through this problem, $\mean{x}$ is a mean over a single sample, and $\left<x \right>$ is average over many samples.
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\section{Solution} \label{sec:solution}
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\subsection{(a) Simplify $P$} \label{subsec:sola}
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\begin{align}
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P(\left[x_n \right] | \mu, \sigma^2) &= \prod_{n = 1}^N \mathcal{N}(x_n | \mu, \sigma^2) \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \prod_{n = 1}^N \frac{1}{\sqrt{2 \pi \sigma^2}} e^{\flatfrac{-\left(x_n - \mu\right)^2}{\left(2 \sigma^2\right)}} \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N \prod_{n = 1}^N e^{\flatfrac{-\left(x_n - \mu\right)^2}{\left(2 \sigma^2\right)}} \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N e^{\flatfrac{1}{\left(2 \sigma^2\right)}} \prod_{n = 1}^N e^{-\left(x_n - \mu\right)^2} \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N e^{\flatfrac{1}{\left(2 \sigma^2\right)}} e^{-\sum_n^N \left(x_n - \mu\right)^2} \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N e^{\flatfrac{1}{\left(2 \sigma^2\right)}} \exp(-\sum_n^N \left(x_n - \mu\right)^2) \label{eq:backup}
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\end{align}
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So we can isolate the grossness to the sum in the numerator.
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\begin{align}
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\sum_n^N \left(x_n - \mu\right)^2 &= \sum_n^N \left( x_n^2 - 2 \mu x_n + \mu^2 \right) \\
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&= \sum_n^N x_n^2 - 2 \sum_n^N \mu x_n + \sum_n^N \mu^2 \\
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&= \sum_n^N x_n^2 - 2 \mu \sum_n^N x_n + \mu^2 \sum_n^N 1 \\
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&= \sum_n^N x_n^2 - 2 \mu N \mean{x} + N \mu^2
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\end{align}
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The pesky $x_n^2$ reminds us of $S$, so we expand it:
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\begin{align}
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S = \sum_n^N \left(x_n - \mean{x} \right)^2 \\
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= \sum_n^N \left(x_n^2 - 2 x_n \mean{x} + \mean{x}^2 \right) \\
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= \sum_n^N x_n^2 - 2 \sum_n^N x_n \mean{x} + \sum_n^N \mean{x}^2 \\
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= \sum_n^N x_n^2 - 2 \mean{x} \sum_n^N x_n + N \mean{x}^2 \\
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= \sum_n^N x_n^2 - 2 N \mean{x}^2 + N \mean{x}^2 \\
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S = \sum_n^N x_n^2 - N \mean{x}^2,
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\end{align}
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so then
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\begin{align}
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\sum_n^N \left(x_n - \mu\right)^2 &= \sum_n^N x_n^2 - 2 \mu N \mean{x} + N \mu^2 \\
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\sum_n^N \left(x_n - \mu\right)^2 &= S + N \mean{x}^2 - 2 \mu N \mean{x} + N \mu^2 \\
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\sum_n^N \left(x_n - \mu\right)^2 &= S + N \left(\mean{x} - \mu \right)^2
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\end{align}
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Inserting this into \cref{eq:backup}, we get
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\begin{align}
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N e^{\flatfrac{1}{\left(2 \sigma^2\right)}} \exp(-\sum_n^N \left(x_n - \mu\right)^2) \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N e^{\flatfrac{1}{\left(2 \sigma^2\right)}} \exp(- S - N \left(\mean{x} - \mu \right)^2) \\
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P(\left[x_n \right] | \mu, \sigma^2) &= \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N \exp(\frac{- S - N \left(\mean{x} - \mu \right)^2}{2 \sigma^2}) \\
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\end{align}
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\subsection{(b) max likelihood}
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This section is basically just a lot of gross derivatives.
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We want to maximise
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\begin{equation}
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P(\left[x_n \right] | \mu, \sigma^2) = \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N \exp(\frac{- S - N \left(\mean{x} - \mu \right)^2}{2 \sigma^2})
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\end{equation}
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Because $\log$ is monotonic, we can maximise $\log P$:
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\begin{align}
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\log P(\left[x_n \right] | \mu, \sigma^2) &= \log \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N \exp(\frac{- S - N \left(\mean{x} - \mu \right)^2}{2 \sigma^2}) \\
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\log P(\left[x_n \right] | \mu, \sigma^2) &= \log \left( \frac{1}{\sqrt{2 \pi \sigma^2}}\right)^N + \log \exp(\frac{- S - N \left(\mean{x} - \mu \right)^2}{2 \sigma^2}) \\
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\log P(\left[x_n \right] | \mu, \sigma^2) &= N \log \frac{1}{\sqrt{2 \pi \sigma^2}} + \frac{- S - N \left(\mean{x} - \mu \right)^2}{2 \sigma^2} \\
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\log P(\left[x_n \right] | \mu, \sigma^2) &= - \frac{N}{2} \log \sigma^2 + C - \frac{S + N \left(\mean{x} - \mu \right)^2}{2 \sigma^2} \\
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\log P(\left[x_n \right] | \mu, \sigma^2) &= - N \log \sigma + C - \frac{S + N \left(\mean{x} - \mu \right)^2}{2 \sigma^2}
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\end{align}
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First minimise with respect to $\mu$:
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\begin{align}
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\pdv{\log P}{\mu} &= - \pdv{}{\mu} \frac{S + N \left(\mean{x} - \mu \right)^2}{2 \sigma^2},
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\end{align}
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which clearly reaches zero when $\mu = \mean{x}$.
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Next, we look at minimising with respect to $\sigma$:
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\begin{align}
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\pdv{\log P}{\sigma} &= \pdv{}{\sigma }- N \log \sigma + C - \frac{S + N \left(\mean{x} - \mu \right)^2}{2 \sigma^2} \\
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\pdv{\log P}{\sigma} &= - \frac{N}{\sigma} - \pdv{}{\sigma} \frac{S + N \left(\mean{x} - \mu \right)^2}{2 \sigma^2} \\
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\pdv{\log P}{\sigma} &= - \frac{N}{\sigma} - \frac{S + N \left(\mean{x} - \mu \right)^2}{2} \pdv{}{\sigma} \frac{1}{\sigma^2} \\
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\pdv{\log P}{\sigma} &= - \frac{N}{\sigma} - \frac{S + N \left(\mean{x} - \mu \right)^2}{2} \frac{-2}{\sigma^3} \\
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\pdv{\log P}{\sigma} &= - \frac{N}{\sigma} + \frac{S + N \left(\mean{x} - \mu \right)^2}{\sigma^3}
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\end{align}
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\triv we can now sub $\mu = \mean{x}$, giving us
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\begin{align}
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\pdv{\log P}{\sigma} &= - \frac{N}{\sigma} + \frac{S}{\sigma^3}.
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\end{align}
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Minimising,
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\begin{align}
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0 &= - \frac{N}{\sigma_{ML}} + \frac{S}{\sigma_{ML}^3} \\
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\frac{N}{\sigma_{ML}} &= \frac{S}{\sigma_{ML}^3} \\
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\sigma_{ML} &= \sqrt{\frac{S}{N}}
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\end{align}
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\newpage
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\listoftodos
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\end{document}
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