The math support in docutils is a little lacking (and inconsistent). Using the following in ReST:
j = 1, 2, …, N-1
will raise an error: the parser correctly recognizes the ellipses, mapping them to 'ldots'. However, the mathml generator then doesn't recognize that symbol and throws an error. The attached patch defines the back mapping from 'ldots' to ' …' to fix this.
I took the opportunity to also add support for \gets and \to at the same time (which are aliases to \leftarrow and \rightarrow.