Showing what's behind the curtain

Right triangles with integer coordinates

Finding Nim positions consisting of heap sizes n, 2n and 3n for n ≤ 2^{30} that result in a losing game.

Su Doku

Cube digit pairs

Comparing four-sided and six-sided dice

Odd Triplets

Find fractions that cannot be cancelled down (resilient fraction).

Prime Subset Sums

250250

Product-sum numbers