I occurred a problem recently, but, I donot know how to solve it?
Error:
Undefined control sequence.
l.26 $$\eqalignno
{P^{-1}&=P_{1}^{-1}+P_{2}^{-1}\cr P^{-1}\mathhat{x}&=P_{1}^...
My codes:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$\eqalignno{P^{-1}&=P_{1}^{-1}+P_{2}^{-1}\cr P^{-1}\mathhat{x}&=P_{1}^{-1}\mathhat{x}_{1}+P_{2}^{-1}\mathhat{x}_{2}.&\hbox{(7)}}$$
\end{document}