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Numbers n such that (40^n - 1)/39 is prime.
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%I #16 Mar 14 2016 03:37:42

%S 2,5,7,19,23,29,541,751,1277

%N Numbers n such that (40^n - 1)/39 is prime.

%C No other terms for n < 100000. - _Robert Price_, Jan 27 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[(40^#-1)/39]&]

%o (PARI) isok(n) = isprime((40^n-1)/39); \\ _Michel Marcus_, Mar 13 2016

%K hard,more,nonn

%O 1,1

%A _Alexander Adamchuk_, Feb 11 2007

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