Leyland number

Leyland primes

A Leyland prime is a Leyland number that is prime. The first such primes are:

:17, 593, 32993, 2097593, 8589935681, 59604644783353249, 523347633027360537213687137, 43143988327398957279342419750374600193, ...

corresponding to :32+23, 92+29, 152+215, 212+221, 332+233, 245+524, 563+356, 3215+1532.

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, ... ().

By November 2012, the largest Leyland number that had been proven to be prime was 51226753 + 67535122 with digits. From January 2011 to April 2011, it was the largest prime whose primality was proved by elliptic curve primality proving. In December 2012, this was improved by proving the primality of the two numbers 311063 + 633110 (5596 digits) and 86562929 + 29298656 ( digits), the latter of which surpassed the previous record. In February 2023, 1048245 + 5104824 ( digits) was proven to be prime, and it was also the largest prime proven using ECPP, until three months later a larger (non-Leyland) prime was proven using ECPP. There are many larger known probable primes such as 3147389 + 9314738, 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.

Leyland number of the second kind

A Leyland number of the second kind is a number of the form :x^y - y^x where x and y are integers greater than 1. The first such numbers are:

: 0, 1, 7, 17, 28, 79, 118, 192, 399, 431, 513, 924, 1844, 1927, 2800, 3952, 6049, 7849, 8023, 13983, 16188, 18954, 32543, 58049, 61318, 61440, 65280, 130783, 162287, 175816, 255583, 261820, ...

A Leyland prime of the second kind is a Leyland number of the second kind that is also prime. The first few such primes are:

:7, 17, 79, 431, 58049, 130783, 162287, 523927, 2486784401, 6102977801, 8375575711, 13055867207, 83695120256591, 375700268413577, 2251799813682647, ... .

For the probable primes, see Henri Lifchitz & Renaud Lifchitz, PRP Top Records search.

References

References

1. [[Richard Crandall]] and [[Carl Pomerance]]. (2005). "Prime Numbers: A Computational Perspective". Springer.
2. "Primes and Strong Pseudoprimes of the form x^y + y^x". Paul Leyland.
3. "Elliptic Curve Primality Proof". Chris Caldwell.
4. (2012-12-11). "Mihailescu's CIDE". mersenneforum.org.
5. "Leyland prime of the form 104824⁵+5¹⁰⁴⁸²⁴". Prime Wiki.
6. "Elliptic Curve Primality Proof". Prime Pages.
7. Henri Lifchitz & Renaud Lifchitz, [http://www.primenumbers.net/prptop/searchform.php?form=x%5Ey%2By%5Ex&action=Search PRP Top Records search].
8. "Factorizations of x^y + y^x for 1 < y < x < 151". Andrey Kulsha.