I think I read this short story 20, 30 years ago. The story concerns two mathematicians in a dark dystopian future. The one is struggling with some project, and can't figure out how to get the thing done; his friend gives him some advice, involving the use of the forbidden "dark numbers", which include such concepts as zero. No fantasy, I thought is was C.J. Cherryh but I can't find any hits there.
It's driving me nuts, what story was this?
I'm reasonably certain I read this in a collection of short stories, most likely bought from a second-hand book store.
The point of the story was a commentary on the rulers (government? religious?) who forbade the use of mathematics that would allow people to solve significant problems (think engineering and the like) -- not individual people, but rather society at large.