diff --git a/chapters/vectors-matrix-ops/ranks-determinants/reading/README.md b/chapters/vectors-matrix-ops/ranks-determinants/reading/README.md deleted file mode 100644 index 46d1103..0000000 --- a/chapters/vectors-matrix-ops/ranks-determinants/reading/README.md +++ /dev/null @@ -1 +0,0 @@ -# Ranks and Determinants diff --git a/chapters/vectors-matrix-ops/ranks-determinants/reading/read.md b/chapters/vectors-matrix-ops/ranks-determinants/reading/read.md new file mode 100644 index 0000000..fcefec9 --- /dev/null +++ b/chapters/vectors-matrix-ops/ranks-determinants/reading/read.md @@ -0,0 +1,379 @@ +# Ranks and Determinants + +## The Rank of a Matrix + +The rank of an $n \times p$ matrix $A$, denoted by $\text{rank}(A)$, is the largest number of columns of $A$, which are not linearly dependent (i.e. the number of linearly independent columns). + +### Details + +Vectors $a_1, a_2, \ldots, a_n$ are said to be linearly dependent if there exist constants $k_1, \ldots, k_n$ that are not all zero, such that + +$$k_1 a_1 + k_2 a_2 + \ldots + k_n a_n = 0$$ + +Note that if such constants exist, then we can write one of the $a$ 's as a linear combination of the rest, e.g. if $k_1 \neq 0$ then + +$$a_1=\mathbf{c_1} = -\displaystyle\frac{k_2}{k_1} a_2 - \ldots - \displaystyle\frac{k_2}{k_1} a_n$$ + +It can be shown that the rank of $A$, is the same as the rank of $A'$ + +i.e. the maximum number of linearly independent rows of $A$. + +:::note Note + +Note that if $\text{rank}(A) = p$, then the columns are linearly independent. + +::: + +### Examples + +:::info Example + +If + +$$ +A = + \left[ + \begin{array}{cc} + 1 & 0 \\ + 0 & 1 + \end{array} + \right] +$$ + +then $\text{rank}(A)$ = 2, since + +$$ +k_1 + \left( + \begin{array}{cc} + 1 \\ + 0 + \end{array} + \right) + +k_2 + \left( + \begin{array}{cc} + 0 \\ + 1 + \end{array} + \right) = + \left( + \begin{array}{cc} + 0 \\ + 0 + \end{array} + \right) +$$ + +if and only if + +$$ +\left( + \begin{array}{cc} + k_1 \\ + k_2 + \end{array} +\right) = +\left( + \begin{array}{cc} + 0 \\ + 0 + \end{array} +\right) +$$ + +so the columns are linearly independent. + +::: + +:::info Example + +If + +$$ +A = + \left[ + \begin{array}{ccc} + 1 & 0 & 1 \\ + 0 & 1 & 1 \\ + 0 & 0 & 0 + \end{array} + \right] +$$ + +then $\text{rank}(A)$ = 2. + +::: + +:::info Example + +If + +$$ +A = + \left[ + \begin{array}{ccc} + 1 & 1 & 1 \\ + 0 & 1 & 0 \\ + 0 & 1 & 0 + \end{array} + \right] +$$ + +then $\text{rank}(A)$ = 2. since + +$$ +1 + \left( + \begin{array}{ccc} + 1 \\ + 0 \\ + 0 + \end{array} + \right) + +0 + \left( + \begin{array}{ccc} + 0 \\ + 1 \\ + 1 + \end{array} + \right) + +(-1) + \left( + \begin{array}{ccc} + 1 \\ + 0 \\ + 0 + \end{array} + \right) = +0 +$$ + +(and hence the rank cannot be more than 2) but + +$$ +k_1 + \left( + \begin{array}{ccc} + 1 \\ + 0 \\ + 0 + \end{array} + \right) + +k_2 + \left( + \begin{array}{ccc} + 0 \\ + 1 \\ + 1 + \end{array} + \right) +$$ + +if and only if $k_1=k_2=0$ (and hence the rank must be at least 2). + +::: + +## The Determinant + +Recall that for a $2 \times 2$ matrix, + +$$ +A = + \begin{bmatrix} + a & b \\ + c & d + \end{bmatrix} +$$ + +the inverse of $A$ is + +$$ +A^{-1} = \displaystyle\frac{1}{ad-bc} + \begin{bmatrix} + 2 & 3 \\ + 3 & 1 + \end{bmatrix} +$$ + +### Details + +:::note Definition + +The number $ad-bc$ is called the **determinant** of the $2 \times 2$ matrix $A$. + +::: + +:::note Definition + +Now suppose $A$ is an $n \times n$ matrix. +An **elementary product** from the matrix is a product of $n$ terms based on taking exactly one term from each column of row $x$. +Each such term can be written in the form $a_{1j_1} \cdot a_{2j_2} \cdot a_{3j_3} \cdot \ldots \cdot a_{nj_n}$ where $j_1, \ldots, j_n$ is a permutation of the integers $1,2, \ldots, n$. +Each permutation $\sigma$ of the integers $1,2,\ldots,n$ can be performed by repeatedly interchanging two numbers. + +::: + +:::note Definition + +A **signed elementary product** is an elementary product with a positive sign if the number of interchanges in the permutation is even but negative otherwise. + +::: + +The determinant of $A$, $\det(A)$ or $\vert A \vert$, is the sum of all signed elementary products. + +### Examples + +:::info Example + +$$ +A = + \begin{bmatrix} + a_{11} & a_{12} \\ + a_{21} & a_{22} + \end{bmatrix} +$$ + +then + +$\vert A \vert = a_{1\underline{1}} a_{2\underline{2}} - a_{1\underline{2}}a_{2\underline{1}}$. + +::: + +:::info Example + +If + +$$ +A = + \begin{bmatrix} + a_{11} & a_{12} & a_{13} \\ + a_{21} & a_{22} & a_{23} \\ + a_{31} & a_{32} & a_{33} + \end{bmatrix} +$$ + +Then $\vert A \vert$ + += $a_{11} a_{22} a_{33}$ This is the identity permutation and has positive sign + +$-a_{11} a_{23} a_{32}$ This is the permutation that only interchanges $2$ and $3$ + +$-a_{12} a_{21} a_{33}$ Only one interchange + +$+a_{12} a_{23} a_{31}$ Two interchanges + +$+a_{13} a_{21} a_{32}$ Two interchanges + +$-a_{13} a_{22} a_{31}$ Three interchanges + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 1 & 1 \\ + 1 & 0 + \end{bmatrix} +$$ + +$\vert A \vert = -1$ + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 1 & 0 & 0 \\ + 0 & 2 & 0 \\ + 0 & 0 & 3 + \end{bmatrix} +$$ + +$\vert A \vert = 1 \cdot 2 \cdot 3 = 6$ + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 1 & 0 & 0 \\ + 0 & 2 & 0 \\ + 0 & 3 & 0 + \end{bmatrix} +$$ + +$\vert A \vert = 0$ + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 1 & 0 & 0 \\ + 0 & 0 & 2 \\ + 0 & 3 & 0 + \end{bmatrix} +$$ + +$\vert A \vert = -6$ + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 2 & 1 \\ + 2 & 1 + \end{bmatrix} +$$ + +$\vert A \vert = 0$ + +::: + +:::info Example + +$$ +A = + \begin{bmatrix} + 1 & 0 & 1 \\ + 0 & 1 & 1 \\ + 1 & 1 & 2 + \end{bmatrix} +$$ + +$\vert A \vert = 0$ + +::: + +## Ranks, Inverses and Determinants + +The following statements are true for an $n\times n$ matrix $A$ : + +- $\text{rank} (A)= n$ + +- $\det(A)\neq 0$ + +- $A$ has an inverse + +### Details + +Suppose $A$ is an $n\times n$ matrix. +Then the following are truths: + +- $\text{rank} (A)= n$ + +- $\det(A)\neq 0$ + +- $A$ has an inverse