# Generalized Repunit

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

A repunit is a number whose expansion (in radix 10) is a string of ones (for example: 11 and 11111111). A generalized repunit (radix b) is one whose expansion base b is all ones. For example, the Mersenne primes are the generalized repunits in base two (binary). Here is a formula for the n "digit" generalized repunit (base b):
(bn - 1)/(b - 1).
Clearly a generalized repunit prime is a generalized repunit that is prime.

In the special case that b is prime, the generalized repunit is the value of the sum of divisors function: \sigma (bn-1).

Note that every odd prime p is a generalized repunit in base p-1, in fact in that base it is "11". So we do not archive all generalized repunit primes, just those for which the number of digits is at least one fifth of the base (and the base must be positive!) This anthropocentric limit was choosen so that 11 (in base 10) is still considered a generalized repunit (as well as an ordinary repunit).

Below we list the record repunit primes with b > 2. The Mersennes (b=2) have their own pages.

### Record Primes of this Type

rankprime digitswhowhencomment
1(6384713339 - 1)/63846 64091 p170 May 2013 Generalized repunit
2(2637113681 - 1)/26370 60482 p170 Feb 2012 Generalized repunit
3(452916381 - 1)/4528 59886 CH2 Dec 2012 Generalized repunit
4(3828411491 - 1)/38283 52659 CH2 Feb 2013 Generalized repunit
5(3412011311 - 1)/34119 51269 CH2 Sep 2011 Generalized repunit
6(325569283 - 1)/32555 41887 CH2 Apr 2011 Generalized repunit
7(154912973 - 1)/1548 41382 p170 Dec 2010 Generalized repunit
8(288398317 - 1)/28838 37090 CH6 Dec 2006 Generalized repunit
9(436610099 - 1)/4365 36758 x14 Mar 2011 Generalized repunit
10(113797411 - 1)/11378 30056 x14 Dec 2009 Generalized repunit
11(133206997 - 1)/13319 28856 x14 Oct 2010 Generalized repunit
12(34297549 - 1)/3428 26684 x14 Jul 2009 Generalized repunit
13(130965953 - 1)/13095 24506 CH6 Nov 2007 Generalized repunit
14(8911971 - 1)/88 23335 CH2 May 2009 Generalized repunit
15(231515347 - 1)/23150 23333 x14 Oct 2008 Generalized repunit
16(58556121 - 1)/5854 23058 CH1 Nov 2005 Generalized repunit
17(20086781 - 1)/2007 22393 CH6 Oct 2010 Generalized repunit
18(199794933 - 1)/19978 21211 x14 Mar 2011 Generalized repunit
19(94734969 - 1)/9472 19756 CH2 Oct 2008 Generalized repunit
20(142614663 - 1)/14260 19367 x14 Nov 2007 Generalized repunit

### References

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Dubner, Harvey, "Repunit R49081 is a probable prime," Math. Comp., 71:238 (2002) 833--835 (electronic).  (http://dx.doi.org/10.1090/S0025-5718-01-01319-9) MR 1885632 (Abstract available)
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H. Dubner, "Generalized repunit primes," Math. Comp., 61 (1993) 927--930.  MR 94a:11009
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Jaroma, John H., On primes and pseudoprimes in the generalized repunitsCongr. Numer., In "Proceedings of the Fortieth Southeastern International Conference on Combinatorics, Graph Theory and Computing," Vol, 195, 2009.  pp. 105--114, MR 2584289
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Salas, Christian, "Base-3 repunit primes and the Cantor set," Gen. Math., 19:2 (2011) 103--107.  MR 2818401
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