math expression in latex

Question

In fact, I don't know how to write this expression on LaTeX:

And thanks in advance.

Answer 1

Here's a way to do it:

\documentclass{article}
\usepackage[utf8]{inputenc} 
\usepackage{fourier} 
\usepackage[thin, , thinc]{esdiff}

\begin{document}

\[
\dot\sigma = \diffp{\sigma}{{\mathbf x}}\overbrace{\dot{\mathbf x}}^{\diff{\mathbf x}{t}}\bigl(\mkern1.5mu\overbrace{f(\mathbf x, t) + B(t, \mathbf x)\mathbf u}^{\dot{\mathbf x}}\mkern1.5mu \bigr)
\]

\end{document}