In number theory, a Leyland number is a number of the form x^y + y^x, where x and y are integers greater than 1. The first few Leyland numbers are

The requirement that x and y both be greater than 1 is important, since without it every positive integer would be a Leyland number of the form x¹ + 1^x. Also, because of the commutative property of addition, the condition x ≥ y is usually added to avoid double-covering the set of Leyland numbers (so we have 1 < y ≤ x).

The first prime Leyland numbers are

corresponding to

As of June 2008, the largest Leyland number that has been proven to be prime is 2638⁴⁴⁰⁵ + 4405²⁶³⁸ with 15071 digits. From July 2004 to June 2006, it was the largest prime whose primality was proved by elliptic curve primality proving. There are many larger known probable primes such as 91382⁹ + 9⁹¹³⁸², but it is hard to prove primality of large Leyland numbers. Paul Leyland writes on his website: "More recently still, it was realized that numbers of this form are ideal test cases for general purpose primality proving programs. They have a simple algebraic description but no obvious cyclotomic properties which special purpose algorithms can exploit."