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?