In 1850, Thomas Penyngton Kirkman, a mathematician when he wasn’t fulfilling his main responsibility as a vicar in the Church of England, described his “schoolgirl problem”: “Fifteen young ladies in a ...

In 1973, Paul Erdős asked if it was possible to assemble sets of “triples” — three points on a graph — so that they abide by two seemingly incompatible rules. A new proof shows it can always be done.