Ahlswede–Daykin inequality

The Ahlswede–Daykin inequality (Ahlswede & Daykin 1978), also known as the four functions theorem (or inequality), is a correlation-type inequality for four functions on a finite distributive lattice. It is a fundamental tool in statistical mechanics and probabilistic combinatorics (especially random graphs and the probabilistic method).

Source: Wikipedia — Ahlswede–Daykin inequality (CC BY-SA 4.0)