

A004063


Numbers k such that (7^k  1)/6 is prime.
(Formerly M3836)


21




OFFSET

1,1


COMMENTS

Among repunits with bases from 11 to 11, base7 repunits have the lowest relative rate of occurrence of primes so far.  Paul Bourdelais, Feb 23 2010


REFERENCES

J. Brillhart et al., Factorizations of b^n + 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

J. Brillhart et al., Factorizations of b^n + 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
Eric Weisstein's World of Mathematics, Repunit


MATHEMATICA

For[n = 1, n <= 20000, n++, If[PrimeQ[(7^n  1)/6 ], Print[n]]] (* Sam Handler (sam_5_5_5_0(AT)yahoo.com), Aug 09 2006 *)


PROG

(Prime95) PRP=1, 7, 1264699, 1, 0, 0, "6"


CROSSREFS



KEYWORD

nonn,hard


AUTHOR



EXTENSIONS

a(8) discovered Sep 17 2008 by Paul Bourdelais & Eric Purohit  it is a probable prime based on trial factoring to 2.5*10^13 and Fermat base 2 primality test.  Paul Bourdelais, Sep 18 2008


STATUS

approved



