Combinatorics

For more than 50 years, the mathematician Neil Sloane has curated the authoritative collection of interesting and important integer sequences.

Solve this variation of Thomas Kirkman’s famous 1850 puzzle by arranging girls in walking groups. And think fast — the clock is ticking.

A 150-year-old conundrum about how to group people has been solved, but many puzzles remain.

A mathematician who has analyzed card shuffling for decades is tackling one final nemesis: “smooshing.”