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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.[1] They are named after the mathematician Paul Leyland. The first few Leyland numbers are

8, 17, 32, 54, 57, 100, 145, 177, 320, 368, 512, 593, 945, 1124 (sequence A076980 in OEIS).

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 + 1x. Also, because of the commutative property of addition, the condition xy is usually added to avoid double-covering the set of Leyland numbers (so we have 1 < yx).

The first prime Leyland numbers are

17, 593, 32993, 2097593, 8589935681, 59604644783353249, 523347633027360537213687137, 43143988327398957279342419750374600193 ( A094133)

corresponding to

32+23, 92+29, 152+215, 212+221, 332+233, 245+524, 563+356, 3215+1532.[2]

One can also fix the value of y and consider the sequence of x values that gives Leyland primes, for example x2 + 2x is prime for x = 3, 9, 15, 21, 33, 2007, 2127, 3759, ... ( A064539).

By November 2012, the largest Leyland number that had been proven to be prime was 51226753 + 67535122 with 25050 digits. From January 2011 to April 2011, it was the largest prime whose primality was proved by elliptic curve primality proving.[3] In December 2012, this was improved by proving the primality of the two numbers 311063 + 633110 (5596 digits) and 86562929 + 29298656 (30008 digits), the latter of which surpassed the previous record.[4] There are many larger known probable primes such as 3147389 + 9314738,[5] 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."

There is a project called XYYXF to factor composite Leyland numbers.[6]

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References

Richard Crandall and Carl Pomerance (2005), Prime Numbers: A Computational Perspective, Springer
"Primes and Strong Pseudoprimes of the form xy + yx". Paul Leyland. Retrieved 2007-01-14.
"Elliptic Curve Primality Proof". Chris Caldwell. Retrieved 2011-04-03.
"Mihailescu's CIDE". mersenneforum.org. 2012-12-11. Retrieved 2012-12-26.
Henri Lifchitz & Renaud Lifchitz, PRP Top Records search.

"Factorizations of xy + yx for 1 < y < x < 151". Andrey Kulsha. Retrieved 2008-06-24.