cp's OEIS Frontend

A001770 Numbers k such that 5*2^k - 1 is prime.

%I A001770 M1087 N0415 #43 Jul 08 2025 16:26:57
%S A001770 2,4,8,10,12,14,18,32,48,54,72,148,184,248,270,274,420,1340,1438,1522,
%T A001770 1638,1754,1884,2014,2170,2548,2622,2652,2704,13510,21738,25624,41934,
%U A001770 51478,52540,53230,172300,245728,350028,1194164
%N A001770 Numbers k such that 5*2^k - 1 is prime.
%C A001770 A084213(a(n)+1) is in A136539, for all n. - _Farideh Firoozbakht_ and _M. F. Hasler_, Nov 03 2012
%D A001770 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D A001770 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A001770 Y. Gallot, <a href="http://www.utm.edu/research/primes/programs/gallot/index.html">Proth.exe: Windows Program for Finding Large Primes</a>
%H A001770 Wilfrid Keller, <a href="http://www.prothsearch.com/riesel2.html">List of primes k.2^n - 1 for k < 300</a>
%H A001770 H. C. Williams and C. R. Zarnke, <a href="http://dx.doi.org/10.1090/S0025-5718-1968-0227095-2">A report on prime numbers of the forms M = (6a+1)*2^(2m-1)-1 and (6a-1)*2^(2m)-1</a>, Math. Comp., 22 (1968), 420-422.
%H A001770 <a href="/index/Pri#riesel">Index entries for sequences of n such that k*2^n-1 (or k*2^n+1) is prime</a>
%p A001770 A001770:=n->`if`(isprime(5*2^n-1),n,NULL): seq(A001770(n), n=1..1000); # _Wesley Ivan Hurt_, Oct 15 2014
%t A001770 Select[Range[1000], PrimeQ[5*2^# - 1] &] (* _Vaclav Kotesovec_, Apr 28 2014 *)
%o A001770 (PARI) is(n)=isprime(5*2^n-1) \\ _Charles R Greathouse IV_, Feb 07 2017
%Y A001770 Cf. A002254 (5*2^n+1 is prime), A050522 (primes of the form 5*2^n - 1).
%K A001770 nonn,nice,hard,more
%O A001770 1,1
%A A001770 _N. J. A. Sloane_
%E A001770 More terms from _Hugo Pfoertner_, Jun 23 2004
%E A001770 a(40) from the Wilfrid Keller link by _Robert Price_, Dec 22 2018