2.3. igraph_arpack_function_t — Type of the ARPACK callback function

typedef int igraph_arpack_function_t(igraph_real_t *to, const igraph_real_t *from,
				     long int n, void *extra);

Arguments:

`to`:	Pointer to an `igraph_real_t`, the result of the matrix-vector product is expected to be stored here.
`from`:	Pointer to an `igraph_real_t`, the input matrix should be multiplied by the vector stored here.
`n`:	The length of the vector (which is the same as the order of the input matrix).
`extra`:	Extra argument to the matrix-vector calculation function. This is coming from the `igraph_arpack_rssolve()` or `igraph_arpack_rnsolve()` function.

Returns:

Error code, if not zero, then the ARPACK solver considers this as an error, stops and calls the igraph error handler.