## Review on Measure Theory and Set Theory

This is an evolving note, such that new concepts will constantly be added.

1. Power set $2^S$ is the set of all subsets of $S$
2. Sigma-algebra
• Given a set $\Omega$, a $\sigma$-algebra on $\Omega$ is a collection $\Sigma \subseteq 2^\Omega$ s.t. $\Sigma$ is nonempty and $\Sigma$ is 1) closed under complements; 2) closed under countable unions
• $\Omega \in\Sigma$
• $\emptyset \in\Sigma$
• $\Sigma$ is closed under countable intersections
• Examples of$\sigma$-algebras
• $\{\emptyset, \Omega\}$
• $\{\emptyset, E, E^C, \Omega\}$
• $2^\Omega$
• Any intersection of $\sigma$-algebras is a $\sigma$-algebra
3. Measure
• A measure $\mu$ on $\Omega$ with $\sigma$-algebra $\Sigma$ is a function for all $E$ in $\Sigma$, $\mu: E\to [0,\infty]$ s.t. 1) $\mu(\emptyset)=0$; 2) $\mu(\bigcup\limits_{k=1}^{\infty}E_i) =\sum\limits_{k=1}^{\infty}\mu(E_i)$ for all countable collections $\{E_i\}^\infty_{i=1}$ of pairwise disjoint sets in $\Sigma$
• A probability measure is a measure $P$ s.t. $P(\Omega)=1$

## Review on Information Theory

This is an evolving note, such that new concepts will constantly be added.

1.  Entropy
• Entropy $\mathrm{H}$ of a discrete random variable $X$ with possible values ${x_1, ..., x_n}$ and probability mass function $P(X)$ is: $\mathrm{H}(X) = \mathbb{E}[\mathrm{I}(X)] = \mathbb{E}[-\log_b{P(X)}] = - \sum\limits_{i=1}^nP(x_i)\log_b{P(x_i)}$
2. Kullback–Leibler divergence (relative entropy)
• For discrete probability distributions $P$ and $Q$, the Kullback–Leibler divergence from $Q$ to $P$ is defined to be: $D_{KL}(P\|Q) = \sum\limits_{i}P(x_i)\log_b{\frac{P(x_i)}{Q(x_i)}}$

## Review on Calculus

This is an evolving note, such that new concepts will constantly be added.

1. Derivatives
• $\frac{d}{dx}cx = c$
• $\frac{d}{dx}x^n = nx^{n-1}$
• $\frac{d}{dx}f(x)+g(x) =f'(x)+g'(x)$
• $\frac{d}{dx}f(x)g(x) = f'(x)g(x) + f(x)g'(x)$
• $\frac{d}{dx}\frac{f(x)}{g(x)} = \frac{f'(x)g(x)-f(x)g'(x)}{g(x)^2}$
• $\frac{d}{dx}e^x = e^x$
• $\frac{d}{dx}log_a(x) = \frac{1}{x\ln(a)}$
• $\frac{d}{dx}a^x = a^x\ln(a)$
• $\frac{d}{dx} \sin(x) = \cos(x)$
• $\frac{d}{dx} \cos(x) = -\sin(x)$
• $\frac{d}{dx} \tan(x) = \sec^2(x)$
• $\frac{dy}{dx} = \frac{dy}{du}\frac{du}{dx}$
2. Matrix calculus
• Derivative: for $f:\mathbb{R}^{m\times n}\to\mathbb{R}$, $(\frac{\partial f(A)}{\partial A})_{i,j} = \frac{\partial f(A)}{\partial A_{i,j}}$ where $A,\frac{\partial f(A)}{\partial A} \in \mathbb{R}^{m \times n}$
• Hessian: for $f:\mathbb{R}^m \to \mathbb{R}$, $\mathbf{H}_{i,j} = \frac{\partial^2 f}{\partial x_i \partial x_j}$ where $\mathbf{H}\in \mathbb{R}^{m\times m}$
3. Taylor series
• $f(x) = f(c) + \frac{f'(c)}{1!}(x-c) + \frac{f''(c)}{2!}(x-c)^2 + ... + \frac{f^{(n)}(c)}{n!}(x-c)^n ... = \sum\limits_{n=0}^{\infty}\frac{f^{(n)}(c)}{n!}(x-c)^n$ for some constant $c$
• n-th taylor polynomial $P_n(x) =\sum\limits_{i=0}^{n}\frac{f^{(i)}(c)}{i!}(x-c)^i$
• error of the n-th taylor polynomial $R_n(x) = f(x) - P_n(x)$
• Lagrange error bound: if $|f^{(n+1)}(x)|\le M$ on an interval $(a-r, a+r)$ for some $r > 0$, then $|R_n(x)|\le M\frac{|x-a|^{n+1}}{(n+1)!}\le M\frac{r^{n+1}}{(n+1)!}$ for all $x\in(a-r,a+r)$.
4. Exponentials
• Properties
• $(x^a)^b = x^{ab}$
• $x^a\cdot x^b = x^{a+b}$
• $\frac{x^a}{x^b} = x^{a-b}$
• Lambert W Function is the inverse relation of the function $f(x) = xe^x$, so $W(xe^x) = x$ or $W(xe^x)e^{w(xe^x)}=f(x)$
• $W(0) = 0$
• $W(1)=\Omega\approx 0.5671$
• $W(e) = 1$
5. Logarithms
• Properties
• $\log(x^a) = a\log(x)$
• $\log_a(x) = \frac{\log_b(x)}{\log_b(a)}$
• $\log(xy) = \log(x) + \log(y)$
• $\log(\frac{x}{y}) = \log(x) - \log(y)$
• $\log_a(a^x) = x$
• $\log(1) = 0$
• for non-negative $A, B, C$, $A \le B + C\sqrt{A} \Rightarrow A \le B + C^2 + C\sqrt{B}$
• Taylor expansion inequalities:
• $e^x \ge 1 + x$
• $e^x \le 1 + x + \frac{x^2}{2}$ for $x \le 0$
• $e^x \le 1 + x + x^2$ for $x \le 1$
• $e^x + e^{-x} \le 2e^{x^2}$
• $\frac{1}{2}e^x +\frac{1}{2}e^{-1x} \le e^{\frac{1}{2}x^2}$

## Review on Probabilities

This is an evolving note, such that new concepts will constantly be added.

1.  Events
• $\mathbb{P}[A\ or\ B] = \mathbb{P}[A\bigcup B] = \mathbb{P}[A] + \mathbb{P}[B] - \mathbb{P}[A \bigcap B] \le \mathbb{P}[A] + \mathbb{P}[B]$
• $\mathbb{P}[A\ and\ B] = \mathbb{P}[A\bigcap B]$
• If $A \Rightarrow B$, then $\mathbb{P} \le \mathbb{P}[B]$
2.  Expectations
• $E[X] = \int xf(x)dx$
• $E[c] = c$
• $E[X + c] = E[X] + c$
• $E[cX] = cE[X]$
• $E[X+Y] = E[X] + E[Y]$
• $E[XY] = E[X]E[Y]$ if $X$ and $Y$ are independent
• $E[\vec{X}] = (E[X_1] \ E[X_2] \ ... \ E[X_d])^T$
• If $X \ge 0$, $E[X] =\int_0^\infty \mathbb{P}[X\ge 0]dt$
• Law of large number: for infinite sequence of i.i.d. random variable $X$, sample mean $\bar{X}_n = \frac{1}{n}(X_1+X_2+...+X_n)$ converges to $E[X_i]$ as $n$ approaches $\infty$
3. Conditional Probabilities and Conditional Expectations
• $\mathbb{P}[A|B] = \frac{\mathbb{P}[A,B]}{\mathbb{P}[B]}$
• Since $\mathbb{P}[A_n,...,A_1] = \mathbb{P}[A_n|A_{n-1},...,A_1] \cdot \mathbb{P}[A_{n-1},...,A_1]$, $\mathbb{P}[\bigcap\limits_{k=1}^nA_k] = \prod\limits_{k=1}^n\mathbb{P}[A_k|\bigcap\limits_{j=1}^{k-1}A_j]$
• Conditional density of $X$ given $Y$: $f_{X|Y}(x|y) = \frac{f_{X,Y}(x,y)}{f_Y(y)}$
• Conditional expectation of $X$ given $Y$: $E[X|Y=y] = \int^\infty_{-\infty} x f_{X|Y}(x|y)dx$
• Law of Total Expectation $E[X] = E_Y[E_{X|Y}[X|Y]]$
4. Variance
• $Var(X) = \int(x-\mu)^2f(x)dx$
• $Var(X) = E[(X - E[X])^2]$
• $Var(X) = E[X^2] - (E[X])^2$
• $Var(c) = 0$
• $Var(X + c) = Var(X)$
• $Var(cX) = c^2Var(X)$
• $Var(X+Y) = Var(X)Var(Y)$ if $X$ and $Y$ are independent
• $Var(\sum\limits_{i=1}^NX_i) = \sum\limits_{i=1}^NVar(X_i)$ if the $X_i...X_n$ are independent
5. Covariance
• $Cov(X,Y) = \int\int(x-E[x])(y-E[y])f(x,y)dxdy$
• $Cov(X,Y) = E[(X - E[X])(Y - E[Y])] = E[XY] - E[X][Y]$
• $Var(X) = Cov(X,X)$
• $Cov(X,c) = 0$
• $Cov(aX, bY) = abCov(X,Y)$
• $Cov(\vec{X}) = E[(\vec{X} - E[\vec{X}])(\vec{X} - E[\vec{X}])^T] = E[\vec{X}\vec{X}^T] - E[\vec{X}](E[\vec{X}])^T$
• $Cov(\vec{X}|\vec{Y}) = E[(\vec{X} - E[\vec{X}|\vec{Y}])(\vec{X} - E[\vec{X}|\vec{Y}])^T|\vec{Y}]$
6. Mean Square Error: $MSE(\hat{\theta}) = E[(\hat{\theta} - \theta)^2] = E[(\hat{\theta}-E[\hat{\theta}])^2] + (E[\hat{\theta}]-\theta)^2 = Var(\hat{\theta}) + Bias(\hat{\theta})^2$, more detail explanation can be found on wikipedia.
7. Inequalities
• Boole’s inequality (Union bound): for a countable set of events $A_1, A_2,...$, $\mathbb{P}[\bigcup\limits_i A_i] \le \sum\limits_{i} \mathbb{P}[A_i]$
• Jensen: for convex $f$, $f(E[X]) \le E[f(X)]$; for concave $f$, $f(E[X]) \ge E[f(X)]$
• Markov: If $X \ge 0$ then for all $t >0$, $\mathbb[P][X\ge t] \le \frac{E[X]}{t}$
• Chebyshev-Cantelli: for $t>0$, $\mathbb{P}[|X-E[X]|\ge t] \le \frac{Var(X)}{Var(X) + t^2}$
• Chebyshev’s Association: let $f$ and $g$ be nondecreasing (nonincreasing) real-valued functions defined on the real line. If $X$ is a real-valued random variable then, $E[f(X)g(X)] \ge E[f(X)]E[g(X)]$
• $E[f(X)g(X)] \le E[f(X)]E[g(X)]$
• Harris’: extends Chebyshev’s Association to functions $f,g:\mathbb{R}^n\rightarrow \mathbb{R}$
• Chernoff bound: for any $t\in \mathbb{R}$, for any random variable $X$, $\mathbb{P}[X\ge t] = \mathbb{P}[e^{sX}\ge e^{st}] \le \frac{E[s^{sX}]}{e^{st}}$
• Cauchy-Schwarz: if $E[X^2]<\infty$ $E[Y^2]<\infty$, then $|E[XY]|\le \sqrt{E[X^2]E[Y^2]}$
• Hoeffding’s inequality: Let $X$ be a random variable with $E[X]=0, a \le X \le b$. Then for $s>0, E[\exp(sX)] \le \exp(s^2\frac{(b-a)^2}{8})$
• Hoeffding’s tail: Let $X_1,...,X_n$ be independent bounded random variable such that $X_i$ falls in the interval $[a_i,b_i]$ with probability one. Then for any $t>0$, $\mathbb{P}[S_n-E[S_n]\ge t] \le \exp\Big(\frac{-2t^2}{\sum\limits_{i=1}^n(b_i-a_i)^2}\Big)$ and $\mathbb{P}[S_n-E[S_n]\le -t] \le \exp\Big(\frac{-2t^2}{\sum\limits_{i=1}^n(b_i-a_i)^2}\Big)$
• Bernstein’s inequality: Let $X_1,...,X_n$ be independent real-valued random variables with $0$ mean, and assume that $|X_i|\le c$ with probability $1$. Let $\sigma^2 = \frac{1}{n}\sum\limits_{i=1}^n Var[X_i]$. Then, for any $\epsilon > 0$, $\mathbb{P}[\frac{1}{n}\sum\limits_{i=1}^n X_i > \epsilon] \le \exp(-\frac{n\epsilon^2}{2\sigma^2 + 2c\epsilon/3})$.
• when $\sigma^2 < \epsilon$, the upper bound behaves like $\exp(-n\epsilon)$ instead of the $\exp(-n\epsilon^2)$ guaranteed by Hoeffding’s tail.
• McDiarmid’s inequality: let $X_1, X_2,...,X_n$ be i.i.d. random variables, and let $f: X^n \mapsto \mathbb{R}$ be s.t. $\forall i, a_1, a_2, ..., a_n, a'_i, |f(a_1, ..., a_i, ..., a_n) - f(a_1, ..., a'_i, ..., a_n)| \le c_i$. Then $\mathbb{P}[f(x_1, x_2, ..., x_n) - E[f(x_1, x_2,...,x_n)] \ge \epsilon] \le \exp\Big(-\frac{2\epsilon^2}{\sum\limits_{i=1}^n c_i^2}\Big)$.
• Azuma-Hoeffding’s inequality: let $x_0, x_1, ..., x_n$ be a martingale (see definition of martingale below) s.t. $\forall k, |x_k - x_{k-1}| < c_k$ for some constant $c_k$, then $\mathbb{P}[x_n - x_0 \ge \epsilon] \le \exp\Big(- \frac{\epsilon^2}{2\sum\limits_{k=1}^n c_k^2}\Big)$.
8. Variables
• Rademacher random variables: $\sigma_1,...,\sigma_n$ s.t. $\mathbb{P}[\sigma_i = 1] = \mathbb{P}[\sigma_i = -1] = 1/2, \forall i \in [n]$
• Rademacher averages: for a set of vectors $S \subseteq \mathbb{R}^n$, the Rademacher average is defined as $\mbox{Rad}(S) = E_{\sigma}[\sup\limits_{v\in S}\langle v,\sigma \rangle_n]$, where $\langle v, \sigma \rangle_n = \frac{1}{n}\sum\limits_{i=1}^n v_i\sigma_i$
• Martingale is a special sequence of random variables
• A discrete-time martingale is a discrete-time stochastic process $X_1, X_2, ... , X_m$ that satisfies
1. $E[|X_i|] < \infty$
2. Let filtration $\mathcal{F}_i = \sigma(X_1, X_2, ..., X_i)$ then $E[X_{i+1}|\mathcal{F}_i] = X_i$. e.g. $\mathcal{F}_i = \{X_1,X_2,...,X_i\}$
9.  Other
• If $\mathbb{P}[X > t]\le F(t)$, then with probability at least $1 - \delta$, $X\le F^{-1}(\delta)$.
• Moment generating function of a random variable $X$ is $M_X(t) = E[e^{tX}]$ where $t\in\mathbb{R}$

## Review on Linear Algebra

This is an evolving note, such that new concepts will constantly be added.

1. Dot Product
• dot product of two vectors can be seem as linearly transform one to the 1D line defined by the other
• $\vec{v} \cdot \vec{u} = |\vec{v}||\vec{u}|\cos\theta$
• $\vec{v} \cdot \vec{u} = \vec{u} \cdot \vec{v}$
• $c(\vec{v}\cdot\vec{u}) = (c\vec{v})\cdot\vec{u}$
• $(c\vec{v} + d\vec{u})\cdot\vec{w} = (c\vec{v})\cdot\vec{w} + (d\vec{u})\cdot\vec{w}$
• $|\vec{v}|^2 =\vec{v}\cdot\vec{v} = \vec{v}^T\vec{v} =\langle\vec{v},\vec{v}\rangle$
• $\vec{v} \cdot \vec{u} = 0$ if and only if $\vec{v}$ and $\vec{u}$ are orthogonal to each other
2. Matrix Multiplication
• $AB\not= BA$
• $A(B+C) = AB + BC$
• $(A+B)C = AC + BC$
• $c(AB) = (cA)B = A(cB)$
• $A(BC) = (AB)C$
• $AI = A = IA$
• $AA^{-1} = I = A^{-1}A$
• $(AB)^{-1} = B^{-1}A^{-1}$
• $det(AB) = det(A) det(B)$
• $A [\vec{v}_1\:\vec{v}_2\:\vec{v}_3] = [A\vec{v}_1\:A\vec{v}_2\:A\vec{v}_3]$
3. Transpose
• $(A^T)^T = A$
• $(A+B)^T = A^T + B^T$
• $(cA)^T = cA^T$
• $(AB)^T = B^TA^T$
• $(A^{-1})^T = (A^T)^{-1}$
4. Vector Differentiation
• $\frac{d(x^TAx)}{d(x)} = x^T(A^T + A)$
5. Determinants
• determinant of two 2D vectors is the area of the signed parallelogram formed by these two vectors ($\det(\vec{a}\vec{b}) = \left| \begin{array}{cc} \vec{a}_1 & \vec{a}_2 \\ \vec{b}_1 & \vec{b}_2 \end{array} \right|$)
• determinant of three 3D vectors is the signed volume of the parallelepiped formed by these three vectors
• if the determinant of a matrix A is 0, then A is singular. Below are some more properties of determinant of matrix:
• $|aA| = a^d|A|$
• $|AB| = |A||B|$
• $|A| = |A^T|$
• $|A| = \frac{1}{|A^{-1}|}$
6. Norms
• p_norm ($l_p$-norm): for $p \in \mathbb{R}$, $\vec{x} \in \mathbb{R}^n$, $\|\vec{x}\|_p = (\sum\limits_{i=1}^n |x_i|^p)^{\frac{1}{p}}$
• Uniform norm (sup norm): for $\vec{x} = (x_1, x_2, \ldots, x_n)$, $\|x\|_\infty = \max(|x_1|, |x_2|,\ldots, |x_n|)$
7. Rank of a matrix (for real matrix $A\in\mathbb{R}^{m\times n}$)
• $columnrank(A) = rowrank(A) = rank(A)$
• $rank(A) = rank(A^T)$
• $rank(A) \le \min(m,n)$
• $rank(AB) \le \min(rank(A), rank(B))$
• $rank(A+B) \le rank(A) + rank(B)$
• $rank(A^TA) = rank(AA^T) = rank(A)$
8. Symetric Matrices
• A matrix $A$ is positive semidefinite if for all vectors $\vec{v}$ such that $\vec{v}^TA\vec{v} \ge 0$
• $A$ is positive semidefinite iff $A=A^T$
• $A=U^TU$ is positive semidefinite
9. Trace (for square matrix)
• $tr(A) = \sum\limits_{i=1}^nA_{ii}$ where $A\in\mathbb{R}^{n\times n}$
• $tr(A) = tr(A^T)$
• $tr(A+B) = tr(A) + tr(B)$
• $tr(cA) = ctr(A)$
• $tr(AB) = tr(BA) = tr(B^TA^T) = tr(A^TB^T)$
10. Linear Transformation
11. Invertibility of a matrix
• A matrix is invertible iff it is full rank
12. Orthogonal Matrix
• “an orthogonal matrix is a square matrix with real entries whose columns and rows are orthogonal unit vectors” (from wikipedia)
• “The rows of an orthogonal matrix are an orthonormal basis. That is, each row has length one, and are mutually perpendicular. Similarly, the columns are also an orthonormal basis. In fact, given any orthonormal basis, the matrix whose rows are that basis is an orthogonal matrix. It is automatically the case that the columns are another orthonormal basis.” (from Wolfram MathWorld)
• Let $A$ and $B$ be orthogonal matrices:
• $A^TA = I = AA^T$
• $\|A\vec{v}\|_2 = \|\vec{v}\|_2$
• $\langle \vec{v},\vec{u} \rangle = \langle A\vec{v},A\vec{u} \rangle$
• $\det(A)=\pm 1$ (when it equals $+1$, $A$ is a rotation matrix; o.w. $A$ is a reflection matrix)
• $A + B$ and $A^{-1}$ are both orthogonal matrices
13. Eigenvectors and Eigenvalues
1. when a transformation only scale or reverse the vector but doesn’t change the direction of the vector (except reverse), we say the vector is a eigenvector of the transformation
2. $A\vec{v} = \lambda\vec{v}$ where $\lambda$ is the eigenvalue associates with the eigenvector $\vec{v}$
3. when we assume there is at least one eigenvector, we can use this equation to find it: $\det(\lambda I - A) = 0$
4. eigenvalue decomposition
•  Definition: “Let $P$ be a matrix of eigenvectors of a given square matrix $A$ and $D$ be a diagonal matrix with the corresponding eigenvalues on the diagonal. Then, as long as $P$ is a square matrix, $A$ can be written as an eigen decomposition $A = PDP^{-1}$. Furthermore, if A is symmetric, then the columns of P are orthogonal vectors.” (from Wolfram MathWorld)
5. Properties
• $tr(A) = \sum\limits_{i=1}^n\lambda_i$
• $|A| = \prod\limits_{i=1}^n\lambda_i$
14. Singular Value Decomposition