Two Short Proofs of the $A^A$ Theorem

Source abstract

In 1984, D.~Duffus asked whether $A^A\cong B^B$ implies $A\cong B$ for finite, nonempty ordered sets $A$ and~$B$. J.\,D. Farley and the author proved that it does. In this paper, we give two short proofs using all intervals of maximum height and the elements with exactly one lower cover. The first proof averages over these intervals; the second takes their disjoint sum and constructs three invariants. Both avoid the auxiliary ordering, direct factorization, and canonical coordinates of the earlier proof.