遇到Undefined control sequence问题

原来是\bm{\theta}无法识别

解决方法:

To use \boldsymbol (in math mode, of course), you need to load the  package.

Alternatively, load the  ("bold math") package and write \bm{\beta} (again in math mode, of course).

e,g,

\documentclass{
     article} \usepackage{ amsmath} % for \boldsymbol macro \usepackage{ bm} % for \bm macro \begin{ document} $\boldsymbol{ \beta} = (\beta_1,\beta_2,\dots,\beta_n)$ $\bm{ \beta} = (\beta_1,\beta_2,\dots,\beta_n)$ \end{ document}