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 A001770 Numbers k such that 5*2^k - 1 is prime. (Formerly M1087 N0415) 11
 2, 4, 8, 10, 12, 14, 18, 32, 48, 54, 72, 148, 184, 248, 270, 274, 420, 1340, 1438, 1522, 1638, 1754, 1884, 2014, 2170, 2548, 2622, 2652, 2704, 13510, 21738, 25624, 41934, 51478, 52540, 53230, 172300, 245728, 350028, 1194164 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A084213(a(n)+1) is in A136539, for all n. - Farideh Firoozbakht and M. F. Hasler, Nov 03 2012 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Wilfrid Keller, List of primes k.2^n - 1 for k < 300 H. C. Williams and C. R. Zarnke, A report on prime numbers of the forms M = (6a+1)*2^(2m-1)-1 and (6a-1)*2^(2m)-1, Math. Comp., 22 (1968), 420-422. MAPLE A001770:=n->`if`(isprime(5*2^n-1), n, NULL): seq(A001770(n), n=1..1000); # Wesley Ivan Hurt, Oct 15 2014 MATHEMATICA Select[Range[1000], PrimeQ[5*2^# - 1] &] (* Vaclav Kotesovec, Apr 28 2014 *) PROG (PARI) is(n)=isprime(5*2^n-1) \\ Charles R Greathouse IV, Feb 07 2017 CROSSREFS Cf. A002254 (5*2^n+1 is prime), A050522 (primes of the form 5*2^n - 1). Sequence in context: A039011 A132190 A155037 * A032494 A125953 A308357 Adjacent sequences:  A001767 A001768 A001769 * A001771 A001772 A001773 KEYWORD nonn,nice,hard,more AUTHOR EXTENSIONS More terms from Hugo Pfoertner, Jun 23 2004 a(40) from the Wilfrid Keller link by Robert Price, Dec 22 2018 STATUS approved

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Last modified July 16 23:49 EDT 2019. Contains 325092 sequences. (Running on oeis4.)