\documentclass{article}
\begin{document}
$$\underset {{x}_1,{h}_1,{m}_2,{b}_2}{max} {u}_1(x)_1-{h}_1+{\beta}_1{W}_2({m}_2,{b}_2)$$
\end{document}
This is the problem I met, sublime text can display the equation but when I run this code, a warning "Undefined control sequence. [$$\underset]"
showed.