# Difference between revisions of "Jacobi matrix"

A Jacobi matrix is a square matrix $[a_{i,k}]$ with real entries such that $a_{i,k} = 0$ for $\left|i-k\right|>1$. If one writes $a_{i,i} = a_i$ ($i=1,\ldots,n$), $a_{i,i+1}=b_i$, and $a_{i+1,i}=c_i$ ($i=1,\ldots,n-1$), then a Jacobi matrix has the form $\left[ \begin{array}{cccccc} a_1 & b_1 & 0 & \cdots & 0 & 0 \\ c_1 & a_2 & b_2 & \cdots & 0 & 0 \\ 0 & c_2 & a_3 & \cdots & 0 & 0 \\ \vdots & \vdots & \vdots & \ddots & \vdots & \vdots \\ 0 & 0 & 0 & \cdots & a_{n-1} & b_{n-1} \\ 0 & 0 & 0 & \cdots & c_{n-1} & a_n \end{array} \right]$ Any minor of a Jacobi matrix $J$ is the product of certain principal minors of $J$ and certain elements of $J$. A Jacobi matrix $J$ is completely non-negative (that is, all its minors are non-negative) if and only if all its principal minors and all elements $b_i$ and $c_i$ ($i=1,\ldots,n-1$) are non-negative. If $b_ic_i>0$ for $i=1,\ldots,n-1$, then the roots of the characteristic polynomial of $J$ are real and distinct.