A098438 - OEIS

A098438

Numbers k such that (30^k - 1)/29 is prime.

2, 5, 11, 163, 569, 1789, 8447, 72871, 78857, 82883

MATHEMATICA

Do[If[PrimeQ[(30^n - 1)/29], Print[n]], {n, 1, 10000}] (* Ryan Propper, Jun 25 2005 *)

Select[Prime[Range[100]], PrimeQ[(30^#-1)/29]&] (* Alexander Adamchuk, Feb 11 2007 *)

PROG

(Magma) for i in [1..500] do if i mod 50 eq 0 then print "counter equals", counter; end if; if IsPrime(i) then n := 0; for j in [0..i-1] do n +:= 30^j; end for; if IsPrime(n) then print n; print i; end if; end if; end for;

(PARI) is(n)=n=(30^n-1)/29; denominator(n)==1&&ispseudoprime(n) \\ Charles R Greathouse IV, Jul 01 2013

CROSSREFS

Searching in the OEIS for 'repunit' gives many similar sequences.

EXTENSIONS

a(5)-a(7), corresponding to probable primes, from Ryan Propper, Jun 25 2005

Edited by N. J. A. Sloane Jan 25 2008 at the suggestion of Herman Jamke (hermanjamke(AT)fastmail.fm)