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 A268061 Numbers n such that 7*8^n - 1 is prime. 4
 3, 7, 15, 59, 6127, 8703, 11619, 23403, 124299 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(10) > 2*10^5. Terms are A001771(n)/3 that are integers. REFERENCES R. K. Guy, Unsolved Problems in Theory of Numbers, Section A3. LINKS H. C. Williams, The primality of certain integers of the form 2Ar^n - 1, Acta Arith. 39 (1981), 7-17. MATHEMATICA Select[Range[0, 200000], PrimeQ[7*8^# - 1] &] PROG (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime(7*8^n-1), print1(n, ", "))) \\ Altug Alkan, Jan 25 2016 CROSSREFS 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). Cf. A001771, A002235, A005541, A247260. Sequence in context: A146448 A192169 A290470 * A298358 A023370 A181071 Adjacent sequences:  A268058 A268059 A268060 * A268062 A268063 A268064 KEYWORD nonn,hard,more AUTHOR Robert Price, Jan 25 2016 STATUS approved

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Last modified December 9 19:51 EST 2019. Contains 329879 sequences. (Running on oeis4.)