我在编译一些数学运算时遇到错误

Question

使用时\include{selectors.tex}

像这样一行：

Suppose $\langle x_n \rangle _{n \in \nat}$ is a rapidly converging sequence of
converges to some irrational number.

我收到此错误：

! Undefined control sequence.
l.22 Suppose $\langle x_n \rangle _{n \in \nat
                                              }$ is a rapidly converging seq...

我真的不知道这到底是什么问题？