\geqslant Undefined control sequence in Latex

When I compile the following latex code:

\begin{align}   
f_{m}(x) = c_{1,m} \times I(x_{i}^{m} \geqslant t_{m} ) + c_{2,m} \times I(x_{i}^{m} <     t_{m} ),  
\end{align} 

I always get an error:

! Undefined control sequence.  
<argument> ..._{1,m} \times I(x_{i}^{m} \geqslant   
   开发者_运维知识库                                           t_{m} ) + c_{2,m} \times I...  
l.124 \end{align}

When I replace \geqslant with >=, it is okay.

I already include the amsmath package. What would cause the problem?

Thanks and regards!

---

That command isn't defined in the amsmath package, but rather the amssymb package -- put \usepackage{amssymb} in your preamble and you should be fine.