LaTeX symbol formatting

Question

\int_{\mathbb R^D} \Delta S \cdot S \, dx 
 = -\int_{\mathbb R^D} \nabla S\cdot \nabla S\, dx  
 = \mean{(\nabla S)^2}

I want to write average symbol on $(\nabla S)^2$ but it is not workig. How can I write?

Answer 1

You probably mean:

\int_{\mathbb R^D} \Delta S \cdot S \, dx 
= -\int_{\mathbb R^D} \nabla S\cdot \nabla S\, dx  
= \overline{(\nabla )^2 S} 

or, as is suggested in comment

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}

\def\mean#1{\left< #1 \right>}

\[
\int_{\mathbb R^D} \Delta S \cdot S \, dx = -\int_{\mathbb R^D} \nabla S\cdot \nabla S\, dx  = \mean{(\nabla S)^2}
\]

\end{document}

with the following result: