# problem in equation beginning [closed]

I wrote the code of the equation but I have an error while writing Jij, any help?

$$J_i_j = \left\{ \begin{array}{l l} \frac{1}{k}exp\left(-\frac{{||x_i - x_j||}^{2}}{{2a}^{2}}\right) \quad x_i is nearest neighbor of x_j,\ 0 \quad \text{else} \end{array} \right.$$

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## closed as too localized by Werner, Thorsten, Qrrbrbirlbel, Marco Daniel, Hendrik VogtApr 6 '13 at 7:10

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Use J_{ij}... – Werner Apr 6 '13 at 4:28
Hi, it is better to visit our chat room first. – kiss my armpit Apr 6 '13 at 4:29
Thanks alot, it worked – SMH Apr 6 '13 at 4:30

I have to use J_{ij} instead of J_i_j thanks to Werner.
 $$J_{ij} = % group subscript with braces \begin{cases} % use cases macro for piecewise functions \frac1K\exp % capital K, \exp, you don't have to type braces around single characters \left(-\dfrac{\|x_i - x_j\|^2} {2a^2}\right) % \dfrac for display fraction bigger ,\| for double |, less braces & x_i \text{ is nearest neighbour of } x_j,\\ % use & in cases mode, 0 & \text{else} % \text{} \end{cases}$$