# Mersenne cofactor

The Prime Pages keeps a list of the 5000 largest known primes, plus a few each of certain selected archivable forms and classes. These forms are defined in this collection's home page. This page is about one of those forms. Comments and suggestions requested.

### Definitions and Notes

Mersenne primes have always been the most sought after primesEuclid famously connected them to perfect numbers in his Elements over two millennia ago.  Today tens of thosands of individuals search for the next Mersenne prime as part of GIMPS.

When searching for new Mersennes, you should always first look for small divisors (called Mersenne divisors), before applying the definitive (but time consuming) Lucas-Lehmer test.  These divisors must have a very special form as Fermat and Euler proved:

Theorem:
Let p and q be odd primes. If p divides Mq, then p = 1 (mod q) and p = +/-1 (mod 8).
Sometimes, just the fact that a number divides a Mersenne is enough to show it is prime:
Theorem.
Let p = 3 (mod 4) be prime. 2p+1 is also prime if and only if 2p+1 divides Mp.

Well call a prime a Mersenne cofactor if it is a proper divisor of a Mersenne number Mp with p prime.  Below we list the twenty largest known Mersenne cofactors.

### Record Primes of this Type

rankprime digitswhowhencomment
1(263703 - 1)/42808417 19169 c59 Jan 2014 Mersenne cofactor, ECPP
2(258199 - 1)/237604901713907577052391 17497 c59 Jan 2015 Mersenne cofactor, ECPP
3(257131 - 1)/61481396117165983261035042726614288722959856631 17152 c59 Dec 2015 Mersenne cofactor, ECPP
4(253381 - 1)/15588960193 / 38922536168186976769 / 155991271597169062945033668006103 16008 c84 Mar 2017 Mersenne cofactor, ECPP
5(241681 - 1)/1052945423 / 16647332713153 / 2853686272534246492102086015457 12495 c77 Apr 2015 Mersenne cofactor, ECPP
6(241521 - 1)/41602235382028197528613357724450752065089 12459 c54 Aug 2012 Mersenne cofactor, ECPP
7(241263 - 1)/(1402943 · 983437775590306674647) 12395 c59 Oct 2012 Mersenne cofactor, ECPP
8(235339 - 1)/490988430384989040283954404\
86235033667674267835480981233904512709297747031041
10562 c77 Apr 2015 Mersenne cofactor, ECPP
9(232531 - 1)/(65063 · 25225122959) 9778 c60 Sep 2012 Mersenne cofactor, ECPP
10(229473 - 1)/(5613392570256862943 · 24876264677503329001) 8835 c59 Sep 2012 Mersenne cofactor, ECPP
11(228771 - 1)/104726441 8653 c56 Feb 2012 Mersenne cofactor, ECPP
12(228759 - 1)/226160777 8649 c60 Sep 2012 Mersenne cofactor, ECPP
13(226903 - 1)/1113285395642134415541632833178044793 8063 c55 Dec 2011 Mersenne cofactor, ECPP
14(225933 - 1)/1343522383641330719274248287 / 5589137403017310421606050379256829183569 7740 c86 Jan 2017 Mersenne cofactor
15(225243 - 1)/252431 / 403889 / 43014073 / 449245236879223161338352589831 7551 c84 Dec 2016 Mersenne cofactor, ECPP
16(222193 - 1)/1482314857 / 335842152520679489 / 550120409183543541076906984732919 6622 c90 Apr 2018 Mersenne cofactor
17(220887 - 1)/(694257144641 · 3156563122511 · 28533972487913 · 1893804442513836092687) 6229 c4 Sep 2009 Mersenne cofactor, ECPP
18(220521 - 1)/123127 / 2515394736937 / 1963491988082957280584769807344972887 6124 c84 Oct 2017 Mersenne cofactor, ECPP
19(219121 - 1)/917809 / 415147656569 / 1531543915081 / 27784129616513881634842031 5701 c78 Apr 2015 Mersenne cofactor
20(217683 - 1)/(234000819833373807217 · 62265855698776681155719328257) 5274 c4 Sep 2009 Mersenne cofactor, ECPP

### References

Ehrman1967
J. Ehrman, "The number of prime divisors of certain Mersenne numbers," Math. Comp., 21 (1967) 700--704.  MR0223320
EKP1991
P. Erdös, P. Kiss and C. Pomerance, "On prime divisors of Mersenne numbers," Acta. Arith., 57 (1991) 267--281.  MR 92d:11104
Karst1961
E. Karst, "New factors of Mersenne numbers," Math. Comp., 15 (1961) 51.  MR0116481
Karst1962
E. Karst, "Search limits on divisors of Mersenne Numbers," Nordisk Tidskr. Informations-Behandling, 2 (1962) 224--227.  MR0166144
Kravitz1961
S. Kravitz, "Divisors of Mersenne numbers 10,000<p<15,000," Math. Comp., 15 (1961) 292--293.  MR0123508
Pomerance1986
C. Pomerance, "On primitive divisors of Mersenne numbers," Acta Arith., 46:4 (1986) 355--367.  MR871278
SK1967
D. Shanks and S. Kravitz, "On the distribution of Mersenne divisors," Math. Comp., 21 (1967) 97--101.  MR0220665
Wagstaff1988
S. S. Wagstaff, Jr., "Divisors of Mersenne numbers," Math. Comp., 40:161 (1983) 385--397.  MR679454