错误显示如下

Question

\documentclass{article}
\input{tcilatex}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{amsmath}

\begin{lemma}
The Binet formulas for these sequences are%
\begin{eqnarray}
P_{k,n}(x) &=&\frac{\alpha ^{n}(x)-\beta ^{n}(x)}{\alpha (x)-\beta (x)}, \TCItag{2.6} \\
Q_{k,n}(x) &=&\alpha ^{n}(x)+\beta ^{n}(x),  \TCItag{2.7} \\
q_{k,n}(x) &=&\frac{\alpha ^{n}(x)+\beta ^{n}(x)}{2},  \TCItag{2.9}
\end{eqnarray}%
\end{lemma}

错误显示如下

Undefined control sequence.
l.194 ...a ^{n}(x)}{\alpha (x)-\beta (x)}, \TCItag                                              {2.6} \\

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