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# combinatorics

## The Connoisseur of Number Sequences

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

## The Nine Schoolgirls Challenge

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

## A Design Dilemma Solved, Minus Designs

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

## For Persi Diaconis’ Next Magic Trick …

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