TeX source:

p_{i j}^{k}= \begin{cases}\frac{\left[\tau_{i}\right]^{\alpha}\left[\eta_{i j}\right]^{\beta}}{\sum_{l \in N_{l}^{k}}\left[\tau_{i j}\right]^{\alpha}\left[\eta_{i j}\right]^{\beta}} & \text { When } j \in N_{i}^{k} \\ 0 & \text { Else }\end{cases}