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%I
%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
%D H. Dubner, Generalized repunit primes, 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>
%t Select[Prime[Range[100]],PrimeQ[(40^#-1)/39]&]
%Y Cf. A028491, A004061, A004062, A004063, A004023, A005808, A004064, A016054, A006032, A006033, A006034, A006035, A127995, A127996, A127997, A127998, A127999, A128000, A098438, A128002, A128003, A128004.
%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
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