Missing $ inserted in tabular equation environment

Question

Been trying to this lines, but getting the error of Missing $ inserted ,

\begin{equation}
\left \{
\begin{tabular}{ccc}
R_{e_1,...,e_k} & when T_{e_1,...,e_k} > 0 & and BEL(R_{e_1,...,e_k}) \geq \alpha \\
& & \\
M+1& otherwise&\\
\end{tabular}
\right
\end{equation}

Could anyone please tell me why I'm getting This error?

Answer 1

Alternatively, I'd put this in a cases environment (additionally, BEL should be treated like an operator or function instead of the product of three variables B, E, and L; and also using \ldots):

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\bel}{BEL}
\begin{document}
\begin{equation}
x = 
\begin{cases}
R_{e_1,\ldots,e_k} & \text{when } T_{e_1,\ldots,e_k} > 0 \text{ and } \bel(R_{e_1,\ldots,e_k}) \geq \alpha \\
M+1 & \text{otherwise} \\
\end{cases}
\end{equation}
\end{document}

Answer 2

All mathematical expressions, as R_{e_1,...,e_k} shoud be in dollars: $R_{e_1,...,e_k}$ , or better $R_{e_1,\ldots,e_k}$. But much easier and more natural is to change your tabular into array (using amsmath is assumed to have \text macro):

\begin{equation}
\left \{
\begin{array}{ccc}
R_{e_1,\ldots,e_k} & \text{when }T_{e_1,\ldots,e_k} > 0 & \text{ and }BEL(R_{e_1,\ldots,e_k}) \geq \alpha \\
& & \\
M+1& \text{otherwise}&\\
\end{array}
\right.
\end{equation}

Please observe also that there was a missing dot after closing \right (should be \right.).

Answer 3

\documentclass{article}

\usepackage{amsmath}
\begin{document}
\begin{equation}
\left \{
\begin{tabular}{ccc}
$R_{e_1,...,e_k}$ & when $T_{e_1,...,e_k} > 0$ & and $BEL(R_{e_1,...,e_k})\geq \alpha$\\
& & \\
$M+1$& otherwise&\\
\end{tabular}
\right \}
\end{equation}