That looks reasonable to me!

If I am not mistaken, \cdot is the same symbol used for sparse arrays. In that context it means 0 instead of undefined. Personally, I don't think it's a problem: context should make it clear enough. In my specific case, as an inexperienced person just trying to initialize a matrix, I would have been happy with any representation which didn't look like an error. Thus I'm fine with this solution (as I said, I would have been fine with basically everything).