%I #20 Dec 12 2017 01:47:54

%S 2,5,17,457,1423,115877

%N Numbers n such that (28^n - 1)/27 is prime.

%C No other terms less than 100000. - _Robert Price_, May 01 2012

%H H. Dubner, <a href="http://dx.doi.org/10.1090/S0025-5718-1993-1185243-9">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927-930.

%H H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a>

%H <a href="/index/Pri#primepop">Index to primes in various ranges</a>, form ((k+1)^n-1)/k

%t Select[Prime[Range[100]],PrimeQ[(28^#-1)/27]&]

%o (PARI) is(n)=ispseudoprime((28^n-1)/27) \\ _Charles R Greathouse IV_, Jun 13 2017

%K hard,more,nonn

%O 1,1

%A _Alexander Adamchuk_, Feb 11 2007

%E 1423 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008

%E a(6)=115877 corresponds to a probable prime discovered by _Paul Bourdelais_, Dec 08 2017