PsyGraph Games - The world's hardest logic and puzzle games

Ruhr-Universität Bochum - Fakultät für Mathematik

ResearchGate - My profile

OEIS - My profile

LinkedIn - My profile

Matroids Matheplanet - Mathematik Forum (German)

Mathematik alpha - freie Mathematik Software (German)

Dr. Hans Walser - mathematische Homepage mit Schwerpunkt Geometrie (German)

Multimagie - all kinds of magic squares (solutions, proofs, conjectures, open problems)

On the 3x + 1 problem - comprehensive page for the Collatz problem

The Number Theory Web - comprehensive page on topics in number theory