[philiplu]

I do, but there's nothing too obscure in it. Efficient prime number sieve, prime factorization using trial division, generating list of divisors from the prime factorization, modular inverse via Euclidean algorithm, Chinese Remainder Theorem.

I also have some stand-alone modules, one to solve generalized Pell equations, another to find a polynomial given a sequence via the differences (e.g. 2, 5, 10, 17, first differences 3, 5, 7, second 2, 2 is enough to find n^2+1). There's another to find the closed form for a sequence as a linear recurrence.

Some solvers have much more extensive libraries, but I tend to grab bits of code from old solutions to reuse on the fly.