December 2025 - Solution
December 2025 Solution:
The numerical solutions are
f(10^8) = 972989871151789 f(10^9) = 87105187375692805
This riddle was pretty straightforward, and the challenge was in writing an efficient implementation. For the primes, a standard sieve could be used to obtain them. For the sums, we received solutions describing them as convolutions and using Fourier transform techniques which is a nice way to consider this problem.