# How to add bracket under a matrix; two matrix in the same row

matrix:

\begin{bmatrix}
0 & 0 & 0 & 0 & 0& 0 &\cdots \\
0 & 0 & 0 &0 & 0 & 0 &\cdots\\
x_{i,1} &0 &0 & 0 & 0&0 &\cdots \\
0&x_{i,2}&x_{i,1} &0 & 0&0 &\cdots \\
0&0& 0 &x_{i,3} &x_{i,2} &x_{i,1} &\cdots\\
\vdots & \vdots & \vdots &\vdots  &\vdots & \vdots  &\ddots\\
\end{bmatrix}

\begin{bmatrix}
0 & 0 & 0 & 0 &\cdots \\
0 & 0  & 0 &0 &\cdots \\
\Delta x_{i,2} &0  &0 &0 &\cdots \\
0&\Delta  x_{i,3} &0& 0  & \cdots\\
0& 0 & \Delta x_{i,4}&0 &\cdots\\
\vdots & \vdots & \vdots  & \vdots  &\ddots
\end{bmatrix}


I got to know how to add embrace under one matrix from this forum. I hope to generate two matrices with different underneath text. It would be better if there is a larger space between these two matrices. Thanks a million.

With use of the \underbrace{<your matrix>}_{\text{<your text>}}:
\documentclass{article}
$\underbrace{\begin{bmatrix} % <--- 0 & 0 & 0 & 0 & 0& 0 &\cdots \\ 0 & 0 & 0 &0 & 0 & 0 &\cdots\\ x_{i,1} &0 &0 & 0 & 0&0 &\cdots \\ 0&x_{i,2}&x_{i,1} &0 & 0&0 &\cdots \\ 0&0& 0 &x_{i,3} &x_{i,2} &x_{i,1} &\cdots\\ \vdots & \vdots & \vdots &\vdots &\vdots & \vdots &\ddots\\ \end{bmatrix}}_{\text{DIF equation}} % \underbrace{\begin{bmatrix} % <--- 0 & 0 & 0 & 0 &\cdots \\ 0 & 0 & 0 &0 &\cdots \\ \Delta x_{i,2} &0 &0 &0 &\cdots \\ 0&\Delta x_{i,3} &0& 0 & \cdots\\ 0& 0 & \Delta x_{i,4}&0 &\cdots\\ \vdots & \vdots & \vdots & \vdots &\ddots \end{bmatrix}}_{\text{LEV equation}}$