PsyGraph Games – The world’s hardest logic and puzzle games

Ruhr-Universität Bochum – Fakultät für Mathematik

ResearchGate – my profile

OEIS – The On-Line Encyclopedia of Integer Sequences

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

Erich’s Place – Erich Friedman’s awesome mathematical homepage