\input{decls.tex} \title{Linear Algebra} \begin{document} \maketitle \section*{Allowable Operations on a Linear System} Solutions invariant. \begin{itemize} \item Multiply an equation by a non-zero scalar \item Swap two equations \item Add a multiple of one equation to another \end{itemize} \subsection*{Example} \begin{align*} & \systeme{ x - 2y + 2z = 6, -x + 3y + 4z = 2, 2x + y - 2z = -2 } \\\\ E_2 & \implies E_2 + E_1 \\ E_3 & \implies E_3 + E_1 \\ & \systeme{ x - 2y + 2z = 6, y + 6z = 8, 5y - 6z = -14 } \\\\ E_3 & \implies E_3 - 5E_2 \\ & \systeme{ x - 2y + 2z = 6, y + 6z = 8, z = \frac{3}{2} } \\\\ E_1 & \implies E_1 - 2E_3 \\ E_2 & \implies E_2 - 6E_3 \\ & \systeme{ x - 2y = 3, y = -1, z = \frac{3}{2} } \\\\ E_1 & \implies E_1 + 2E_2 \\ & \systeme{ x = 1, y = -1, z = \frac{3}{2} } \\\\ \end{align*} \section*{As Matrices} \begin{align*} \systeme{ x + 2y = 1, 2x - y = 3 } \quad=\quad \begin{pmatrix}[cc|c] 1 & 2 & 1 \\ 2 & -1 & 3 \end{pmatrix} & \systeme{ x - y + z = -2, 2x + 3y + z = 7, x - 2y - z = -2 } \quad=\quad \begin{pmatrix}[ccc|c] 1 & -1 & 1 & -2 \\ 2 & 3 & 1 & 7 \\ 1 & -2 & -1 & -2 \end{pmatrix} \\ \grstep[R_3 - R_1]{R_2 - 2R_1} & \begin{pmatrix}[ccc|c] 1 & -1 & 1 & -2 \\ 0 & 5 & -1 & 11 \\ 0 & -1 & -2 & 0 \end{pmatrix} \\ \grstep{5R_3 + R_2} & \begin{pmatrix}[ccc|c] 1 & -1 & 1 & -2 \\ 0 & 5 & -1 & 11 \\ 0 & 0 & -11 & 11 \\ \end{pmatrix} \\ \grstep{-11^{-1}R_3} & \begin{pmatrix}[ccc|c] 1 & -1 & 1 & -2 \\ 0 & 5 & -1 & 11 \\ 0 & 0 & 1 & -1 \end{pmatrix} \\ \grstep[R_1 - R_3]{R_2 + R_3} & \begin{pmatrix}[ccc|c] 1 & -1 & 0 & -1 \\ 0 & 5 & 0 & 10 \\ 0 & 0 & 1 & -1 \end{pmatrix} \\& \grstep{5^{-1}R_2} & \begin{pmatrix}[ccc|c] 1 & -1 & 0 & -1 \\ 0 & 1 & 0 & 2 \\ 0 & 0 & 1 & -1 \\ \end{pmatrix} \\ \grstep{R_1 + R_2} & \begin{pmatrix}[ccc|c] 1 & 0 & 0 & 1 \\ 0 & 1 & 0 & 2 \\ 0 & 0 & 1 & -1 \end{pmatrix} \\ = & \quad \left\{ \subalign{ x & ~= ~1 \\ y & ~= ~2 \\ z & ~= ~-1 } \right. \end{align*} \section*{Row-Echelon Form} \begin{description} \item[Row-Echelon Form] The leading entry in each row is 1 and is further to the right than the previous row's leading entry, all 0 rows are at the end \item[Reduced Row-Echelon Form] every other entry in a column containing a leading 1 is 0 \item[Theorem:] A matrix can be transformed to reduced row-echelon form using a finite number of allowable row operations \end{description} \subsection*{Example} \begin{align*} & \systeme{3x_1 + 2x_2 = 1, x_1 - x_2 = 4, 2x_1 + x_2 = 5} = \begin{pmatrix}[cc|c] 3 & 2 & 1 \\ 1 & -1 & 4 \\ 2 & 1 & 5 \end{pmatrix} \\ \grstep{R_1\swap R_2} & \begin{pmatrix}[cc|c] 1 & -1 & 4 \\ 3 & 2 & 1 \\ 2 & 1 & 5 \end{pmatrix} \\ \grstep[R_2 - 3R_1]{R_3 - 2R_1} & \begin{pmatrix}[cc|c] 1 & -1 & 4 \\ 0 & 5 & -11 \\ 0 & 3 & -3 \end{pmatrix} \\ \grstep{5^{-1}R_2} & \begin{pmatrix}[cc|c] 1 & -1 & 4 \\ 0 & 1 & \frac{-11}{5} \\ 0 & 3 & -3 \end{pmatrix} \\ \grstep{R_3 - 2R_2} & \begin{pmatrix}[cc|c] 1 & -1 & 4 \\ 0 & 1 & \frac{-11}{5} \\ 0 & 0 & \frac{18}{5} \end{pmatrix} \\ = & \systeme{ x_1 - x_2 = 4, x_2 = \frac{-11}{5}, 0x_1 + 0x_2 = \frac{18}{5} } \end{align*} \begin{align*} & \begin{pmatrix}[cccc|c] 1 & -1 & 1 & 1 & 6 \\ -1 & 1 & -2 & 1 & 3 \\ 2 & 0 & 1 & 4 & 1 \\ \end{pmatrix} \\ \grstep[R_2 + R_1]{R_3 - 2R_1} & \begin{pmatrix}[cccc|c] 1 & -1 & 1 & 1 & 6 \\ 0 & 0 & -1 & 2 & 9 \\ 0 & 2 & -1 & 2 & -11 \end{pmatrix} \\ \grstep[R_2\swap R_3]{2^{-1}R_3} & \begin{pmatrix}[cccc|c] 1 & -1 & 1 & 1 & 6 \\ 0 & 1 & \frac{1}{2} & 1 & \frac{-11}{2} \\ 0 & 0 & -1 & 2 & 9 \\ \end{pmatrix} \\ \grstep[R_1 + R_3]{R_2 - 2^{-1}R_3} & \begin{pmatrix}[cccc|c] 1 & -1 & 0 & 3 & 15 \\ 0 & 1 & 0 & 0 & -10 \\ 0 & 0 & -1 & 2 & 9 \\ \end{pmatrix} \\ \grstep[-R_3]{R_1 + R_2} & \begin{pmatrix}[cccc|c] 1 & 0 & 0 & 3 & 15 \\ 0 & 1 & 0 & 0 & -10 \\ 0 & 0 & 1 & -2 & -9 \\ \end{pmatrix} \\ = & \systeme{ x_1 + 3x_4 = 5, x_2 = -10, x_3 - 2x_4 = -9 } \\ = & \left\{\substack{ x_1 = 5 - 3t \\ x_2 = -10 \\ x_3 = -9 + 2t }\right. \end{align*} \section*{Determinants} The determinant of a matrix is defined only for square matrices. \[\det{A} \neq 0 \iff \exists \text{ a unique solution to the linear system represented by } A\] Let \[A = \begin{pmatrix} a_{11} & a_{12} & a_{1n} \\ a_{21} & \ddots & \vdots \\ a_{31} & \ldots & a_{3n} \\ \end{pmatrix} \] \begin{description} \item[$i, j$ minor of $A$] an $n$x$n$ matrix constructed by removing the $i^\text{th}$ row and $j^\text{th}$ column of $A$ \\ Denoted by $A_{ij}$ \end{description} \begin{align*} & \det{A} \text{ where } n = 1. = a_{11} \\ & \det{A} = a_{11}\det{A_{11}} - a_{12}\det{A_{12}} + ... + (-1)^{n+1}a_{1n} \tag{Laplace expansion of the first row} \\ & \qquad \text{or laplace expansion along other row or column} \text{For } n = 2: & \\ & \det{A} = a_{11}\cdot a_{22} - a_{12}\cdot a_{21} \end{align*} \begin{description} \item[Upper Triangular] lower left triangle is 0 - $d_{ij} = 0 \quad \forall{i > j}$ \item[Lower Triangular] upper right triangle is 0 - $d_{ij} = 0 \quad \forall{i < j}$ \item[Diagonal] only values on the diagonal - $d_{ij} = 0 \quad \forall{i \neq j}$ \\ $\det{A} = \prod^{N}_{i=0}~a_{ij} \forall~\text{ row-echelon }A$ \end{description} \begin{itemize} \item Multiplying a row of a square matrix $A$ by $r$ multiplies $\det{A}$ by $r$ \item Swapping two rows of a square matrix $A$ multiplies $\det{A}$ by $-1$ \item Adding a multiple of a row does not effect the determinant \end{itemize} \section*{Transposition} \begin{description} \item[$A^T$] $a^T_{ij} = a_{ji}~ \forall~i,j$ \end{description} Note: $\det{A} = \det{A^T}~\forall~A$ \section*{Matrix Multiplication} LHS has columns $=$ rows of RHS \\ It's the cartesian product \[A\times B = (a_{i1}b_{j1} + a_{i2}b_{2j} + \ldots + a_{im}b_{mj})_{ij}\] \begin{align*} \begin{pmatrix}[c|c|c] 2 & 1 + 1 & 3 + 6 \\ 4(2) & 4 + 1 & 3(4) + 6 \\ 0 & 2 & 2(6) \\ \end{pmatrix} = \begin{pmatrix} 2 & 2 & 9 \\ 8 & 5 & 18 \\ 0 & 2 & 12 \end{pmatrix} \end{align*} \begin{align*} \begin{pmatrix}1 \\ 2 \\ 3 \end{pmatrix}\begin{pmatrix}1 & 2 & 3 & 4\end{pmatrix} + \begin{pmatrix} 1 & 2 & 3 & 4 \\ 5 & 6 & 7 & 8 \\ 9 & 10 & 11 & 12 \\ \end{pmatrix} \end{align*} \[A\vec{x} = \vec{b}\] where $A$ is the coefficient matrix, $\vec{x}$ is the variables, and $\vec{b}$ is the values of the equations of a linear equation system. \subsection*{Inverse Matrices} The identity matrix exists as $I_n$ for size $n$. \[AA^{-1} = I_n = A^{-1}A \quad \forall~\text{matrices }A \text{ of size } n\] Assume that $A$ has two distinct inverses, $B$ and $C$. \begin{align*} & \text{matrix multiplication is associative} \\ \therefore~ & C(AB) = (CA)B \\ \therefore~ & C I_n = I_n B \\ \therefore~ & C = B \\ & \text{ As $B = C$, while $B$ and $C$ are assumed to be distinct, matrices have no more than one unique inverse by contradiction } \end{align*} Matrices are invertible $\iff \det{A} \neq 0$ \[\det{AB} = \det{A}\det{B}\] \[\therefore~ \det{A}\det{A^{-1}} = \det{I_n} = 1\] \[\therefore~ \det{A} \neq 0 \] \begin{align*} \begin{pmatrix} a & b \\ c & d \end{pmatrix}^{-1} = \frac{1}{ad - bc}\begin{pmatrix} d & -b \\ -c & a \end{pmatrix} \end{align*} \subsubsection*{Computation thereof} \[\det{A} = \sum_{k = 1}^{n}~a_{ik}(-1)^{i+j}\det{A_{ij}} \quad \text{ for any $i$}\] \begin{description} \item[Matrix of Cofactors: $C$] determinants of minors \& signs of laplace expansion \\ ie. $\sum A \odot C = \det{A}$ \item[$\adj{A}$ Adjucate of $A$ =] $C^T$ \end{description} \begin{align*} A & = \begin{pmatrix} 1 & 0 & 1 \\ -1 & 1 & 2 \\ 2 & 0 & 1 \end{pmatrix} \\ C(A) & = \begin{pmatrix} 1 & 5 & -2 \\ 0 & -1 & 0 \\ -1 & -3 & 1 \\ \end{pmatrix} \end{align*} $$ A^{-1} = \frac{\adj{A}}{\det{A}} $$ Gaussian elimination can also be used: augmented matrix with $I_n$ on the right, reduce to reduced row-echelon. If the left is of the form $I_n$, the right is the inverse. If there is a zero row, $\det{A} = 0$, and the $A$ has no inverse. \section*{Linear Transformations} \begin{align*} f: & ~ \R^n \to \R^m \\ f & (x_1, \cdots, x_n) = (f_1(x_1, \cdots, x_n), f_2(x_1, \cdots, x_n), \cdots, f_m(x_1, \cdots, x_n)) \end{align*} $f$ is a linear transformation if \(\forall i.~f_i(x_1, \cdots, x_n)\) is a linear polynomial in $x_1, \cdots, x_n$ with a zero constant term \begin{align*} f(x_1,~ x_2) & = (x_1 + x_2,~ 3x_1 - x_2,~ 10x_2) \tag{is a linear transformation} \\ g(x_1,~ x_2,~ x_3) & = (x_1 x_2,~ x_3^2) \tag{not a linear transformation} \\ h(x_1,~ x_2) & = (3x_1 + 4,~ 2x_2 - 4) \tag{not a linear transformation} \\ \end{align*} \[f: \R^n \to \R^m = \vec{x} \to A\vec{x} \] \[\exists \text{ a matrix $A$ of dimension $n$x$m$ } \forall\text{ linear transforms } f \] \[\forall \text{ matrices $A$ of dimension $n$x$m$ } \exists \text{ a linear transform $f$ of dimension $n$x$m$ such that } f(\vec{x}) = A\vec{x} \] Function composition of linear translations is is just matrix multiplication: \begin{align*} f(\vec{x}) & = A\vec{x} \\ g(\vec{y}) & = B\vec{y} \\ (f\cdot g)(\vec{x}) & = g(f(\vec{x})) = BA\vec{x} \end{align*} A function \(f: \R^n \to \R^m\) is a linear transformation iff: \begin{enumerate} \item $f(\vec{x} + \vec{y}) = f(\vec{x}) + f(\vec{y}) \quad \forall~\vec{x},~\vec{y} \in \R^n $ \item $f(r\vec{x}) = r\cdot f(\vec{x}) \quad \forall~\vec{x} \in \R^n, r \in \R $ \end{enumerate} \subsection*{Building the matrix of a linear transform} \[ f(\vec{x}) = f(x_1\vec{e}_1 + x_2\vec{e}_2) = f(x_1\vec{e}_1) + f(x_2\vec{e}_2) = x_1f(\vec{e}_1) + x_2f(\vec{e}_2) \] \[ A = \begin{pmatrix} f(\vec{e}_1) & f(\vec{e}_2) \end{pmatrix} \] \begin{align*} & \vec{e}_1 = \begin{pmatrix} 1 \\ 0 \end{pmatrix} \\ & \vec{e}_2 = \begin{pmatrix} 0 \\ 1 \end{pmatrix} \\ & \vdots \\ & \forall \vec{x}.~ \vec{x} = \sum_{i}^{n}~\vec{e}_i x_i \end{align*} \subsection*{Composition} \[ \paren{f \cdot g}\paren{\vec{x}} = f(g(\vec{x})) = AB\vec{x} \] where: $f(\vec{x}) = A\vec{x}$, $g(\vec{x}) = B\vec{x}$ \subsection*{Geometry} \begin{description} \item[rotation of $x$ by $\theta$ anticlockwise] \( = R_\theta = \begin{pmatrix} \cos{\theta} & -\sin{\theta} \\ \sin{\theta} & \cos{\theta} \end{pmatrix} \) \item[reflection about a line at angle $\alpha$ from the $x$-axis] \( = T_\alpha = R_{\alpha}T_0R_{-\alpha}\) where \( T_0 = \begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix} \) \item[scaling by $\lambda \in \R$] \( = S_\lambda = \lambda I_n\) \item[Skew by $\alpha$ in $x$ and $\gamma$ in $y$] \( \begin{pmatrix} \alpha & 0 \\ 0 & \gamma \end{pmatrix}\) \end{description} The image of the unit square under the linear transform $A$ is a parallelogram of $(0, 0)$, $(a_{11}, a_{21})$, $(a_{12}, a_{22})$, $(a_{11} + a_{12}, a_{21} + a_{22})$, with area $ \abs{\det{A}} $ \subsection*{Inversion} Inversion of a linear transformation is equivalent to inversion of its representative matrix \subsection*{Eigen\{values, vectors\}} \[ \begin{pmatrix} a & 0 \\ 0 & b \end{pmatrix}\begin{pmatrix} 1 \\ 0 \end{pmatrix} = \begin{pmatrix} a \\ 0 \end{pmatrix} = a\vec{e}_1\] \[ \begin{pmatrix} a & 0 \\ 0 & b \end{pmatrix}\begin{pmatrix} 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 0 \\ b \end{pmatrix} = b\vec{e}_2\] \[ T_\alpha \vec{x} = \vec{x} \text{ for $\vec{x}$ along the line of transformation }\] \begin{description} \item[Eigenvector (of some transformation $f$)] A non-zero vector $\vec{x}$ such that $f(\vec{x}) = \lambda\vec{x}$ for some value $\lambda$ \item[Eigenvalue] $\lambda$ as above \end{description} \[ \forall \text{ eigenvectors of $A$ } \vec{x}, c \in R, \neq 0 .~ c\vec{x} \text{ is an eigenvector with eigenvalue } \lambda\] \[ \forall A: \text{$n$x$n$ matrix}.\quad P_A\paren{\lambda} = \det{\paren{A - \lambda I_n}} \tag{characteristic polynomial in $\lambda$}\] Eigenvalues of $A$ are the solutions of $P_A\paren{\lambda} = 0$ \begin{align*} & A\vec{x} = \lambda\vec{x} & x \neq 0\\ \iff & A\vec{x} - \lambda\vec{x} = 0 \\ \iff & (A - \lambda I_n)\vec{x} = 0 \\ \iff & \det{\paren{A - \lambda I_n}} = 0 \\ & \quad \text{ or $\paren{A - \lambda I_n}$ is invertible and $x = 0$ } \end{align*} \[ P_{R\theta}(\lambda) = \frac{2\cos{\theta} \pm \sqrt{-4\lambda^2\sin^2{\theta}}}{2}\] \[ R_\theta \text{ has eigenvalues }\iff \sin{\theta} = 0 \] \end{document}