In sigma notation, what does it mean when an arrow is used underneath? In particular, in this equation:
$$y_k = f_k\left(\alpha_k + \sum_{j \to k}w_{jk}f_j\left(\alpha_j + \sum_{i \to j}w_{ij}x_i \right)\right)$$
This formula is used to describe a three-layer artificial neural network in machine learning. It’s taken from page 143 of Pattern Recognition and Neural Networks by Brian D. Ripley.
Maybe $i$ and $j$ are nodes of a graph, and $w_{ij}$ is the weight associated with the edge linking $i$ and $j$ ($i \to j$).
In this case, $$\sum_{i\to j} w_{ij}$$ means "the sum over all the $i$ and $j$ such that an edge $i \to j$ exists"