Huntington's axioms

Summary

Huntington's axioms are a short system of axioms for Boolean algebras in terms of one binary operation $+$ and one unary operation $n$. Besides commutativity and associativity of $+$, they need only Huntington's equation $n(n(x) + y) + n(n(x) + n(y)) = x$. Edward Huntington gave this system in 1933, after an earlier set of postulates from 1904. The question whether Huntington's equation can be replaced by a similar equation of Herbert Robbins became the Robbins conjecture.