diff --git a/inputs/lecture_07.tex b/inputs/lecture_07.tex index 86555a5..8a63fd6 100644 --- a/inputs/lecture_07.tex +++ b/inputs/lecture_07.tex @@ -122,7 +122,7 @@ More formally: For every fixed $n$, $Y_n$ and $Z_n$ are independent. \end{claim} \begin{subproof} - This is obvious, but well prove it carefully here. + This is obvious, but we will prove it carefully here. \begin{IEEEeqnarray*}{rCl} &&(\bP \otimes \bP) [Y_n \in (a,b) , Z_n \in (a',b') ]\\ &=& (\bP\otimes\bP) \left( (\omega, \omega') : X_n(\omega) \in (a,b) \land X_n(\omega') \in (a',b') \right)\\ diff --git a/inputs/lecture_09.tex b/inputs/lecture_09.tex index a7b947c..94f1afa 100644 --- a/inputs/lecture_09.tex +++ b/inputs/lecture_09.tex @@ -50,32 +50,38 @@ So far we have dealt with the average behaviour, \[ \frac{\overbrace{X_1 + \ldots + X_n}^{\text{i.i.d.}}}{n} \to \bE(X_1). \] -We now want to understand \vocab{fluctuations} from the average behaviour, +We now want to understand fluctuations from the average behaviour, i.e.\[ X_1 + \ldots + X_n - n \cdot \bE(X_1). \] % TODO improve The question is, what happens on other timescales than $n$? An example is -\[ -\frac{X_1 + \ldots + X_n - n \bE(X_1)}{\sqrt{n} } \xrightarrow{n \to \infty} hv \cN(0, \Var(X_i)) (\ast) -\] -Why is $\sqrt{n}$ the right order? (Handwavey argument) +\begin{equation} + \frac{X_1 + \ldots + X_n - n \bE(X_1)}{\sqrt{n} } + \xrightarrow{n \to \infty} \cN(0, \Var(X_i)) + \label{eqn:lec09ast} +\end{equation} +Why is $\sqrt{n}$ the right order? +Handwavey argument: -Suppose $X_1, X_2,\ldots$ are i.i.d. $\cN(0,1)$. +Suppose $X_1, X_2,\ldots$ are i.i.d.~with $X_1 \sim \cN(0,1)$. The mean of the l.h.s.~is $0$ and for the variance we get \begin{IEEEeqnarray*}{rCl} - \Var(\frac{X_1 + \ldots + X_n - n \bE(X_1)}{\sqrt{n} }) &=& \Var\left( \frac{X_1+ \ldots + X_n}{\sqrt{n} } \right)\\ - &=& \frac{1}{n} \left( \Var(X_1) + \ldots + \Var(X_n) \right) = 1 + \Var(\frac{X_1 + \ldots + X_n - n \bE(X_1)}{\sqrt{n} }) + &=& \Var\left( \frac{X_1+ \ldots + X_n}{\sqrt{n} } \right)\\ + &=& \frac{1}{n} \left( \Var(X_1) + \ldots + \Var(X_n) \right) = 1 \end{IEEEeqnarray*} For the r.h.s.~we get a mean of $0$ and a variance of $1$. -So, to determine what $(\ast)$ could mean, it is necessary that $\sqrt{n}$ +So, to determine what \eqref{eqn:lec09ast} could mean, it is necessary that $\sqrt{n}$ is the right scaling. -To define $(\ast)$ we need another notion of convergence. +To make \eqref{eqn:lec09ast} precise, +we need another notion of convergence. This will be the weakest notion of convergence, hence it is called \vocab{weak convergence}. -This notion of convergence will be defined in terms of characteristic functions of Fourier transforms. +This notion of convergence will be defined in terms of +characteristic functions of Fourier transforms. \subsection{Characteristic Functions and Fourier Transform}