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%I #35 Mar 22 2024 18:37:55
%S 7,17,31,5581,9973,54493,101111,535571
%N Numbers k such that (31^k - 1)/30 is prime.
%C a(5) = 9973 was found by Richard Fischer in 2004. - _Alexander Adamchuk_, Feb 11 2007
%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 R. Fischer, <a href="http://www.fermatquotient.com/PrimSerien/GenRepu.txt">Generalized repunit primes of the form (B^N-1)/(B-1)</a>.
%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[(31^#-1)/30]&]
%o (PARI) is(n)=ispseudoprime((31^n-1)/30) \\ _Charles R Greathouse IV_, Feb 17 2017
%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
%E a(7) = 101111 by _Robert Price_, Nov 26 2011
%E a(8) = 535571 from Alexander Zhirkov, added by _Paul Bourdelais_, Jan 28 2021
%E Missing term 54493 inserted at the suggestion of _Eric Chen_ by _Max Alekseyev_, Aug 26 2023
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