Permutation Websites:
neubert.net
Dr. Neubert's Website: Welcome! Entropy Reduction Laboratory
PSE: Double Shell Periodic System of Elements; FlashSort: O(n) sorting by in situ permutation; Platonic, Geodesic, Spheres; 3D Crystal Structures; 3D...
q-51.com
Q-51 by Wizako. GMAT Hard Math Practice Questions. Free GMAT Sample...
Q-51 series by Wizako is a collection of must solve hard math problems that any serious GMAT test taker should go through. Each question is provided with detailed explanation...