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%I #25 Jan 20 2024 18:57:09
%S 3,7,15,59,6127,8703,11619,23403,124299
%N Numbers k such that 7*8^k - 1 is prime.
%C a(10) > 2*10^5.
%C Terms are A001771(n)/3 that are integers.
%D R. K. Guy, Unsolved Problems in Theory of Numbers, Section A3.
%H H. C. Williams, <a href="http://matwbn.icm.edu.pl/ksiazki/aa/aa39/aa3912.pdf">The primality of certain integers of the form 2Ar^n - 1</a>, Acta Arith. 39 (1981), 7-17.
%t Select[Range[0, 200000], PrimeQ[7*8^# - 1] &]
%o (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime(7*8^n-1), print1(n, ", "))) \\ _Altug Alkan_, Jan 25 2016
%Y Cf. similar sequences of the form k*(k+1)^n-1: A003307 (k=2), ... (k=3), A046865 (k=4), A079906 (k=5), A046866 (k=6), this sequence (k=7), ... (k=8), A056725 (k=9), A046867 (k=10), A079907 (k=11).
%Y Cf. A001771, A002235, A005541, A247260.
%K nonn,hard,more
%O 1,1
%A _Robert Price_, Jan 25 2016
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