Hello,
I noticed that Eigen is getting support for dense tensors (or arrays, if you will), but its support for sparse structures is limited to somewhat specialized matrices and vectors. I recently wrote a general sparse array library, and used that to implement efficient sparse-sparse multiplication (matrix-matrix, matrix-vector, etc). I use it in my project, where most data structures can be thought of as a sparse array. The core idea is to implement sparse array operations based on iterators that iterate through the non-zero elements (and to expose this framework to the user).
I'm inquiring whether there might be interest in incorporating this code in Eigen. The code as it currently stands is at:
-- Elizabeth