

A268110


Numbers k such that (2^kk+1)*2^k+1 is a semiprime.


1



3, 4, 6, 9, 10, 15, 19, 22, 26, 34, 47, 55, 67, 69, 72, 92, 100, 117, 160
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OFFSET

1,1


LINKS

Table of n, a(n) for n=1..19.


EXAMPLE

a(1) = 3 because 6*8 +1 = 49 = 7*7, which is semiprime.
a(2) = 4 because 13*16+1 = 209 = 11*19, which is semiprime.


MAPLE

A268110:=n>`if`(numtheory[bigomega]((2^nn+1)*2^n+1)=2, n, NULL): seq(A268110(n), n=1..80); # Wesley Ivan Hurt, Jan 30 2016


MATHEMATICA

Select[Range[105], PrimeOmega[(2^#  # + 1) 2^# + 1] == 2 &]


PROG

(MAGMA) IsSemiprime:=func<i  &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..105] IsSemiprime(s) where s is (2^nn+1)*2^n+1];
(PARI) lista(nn) = {for(n=1, nn, if(bigomega((2^nn+1)*2^n+1) == 2, print1(n, ", "))); } \\ Altug Alkan, Feb 07 2016


CROSSREFS

Cf. A201360: n for which (2^nn+1)*2^n+1 is prime.
Sequence in context: A245810 A054686 A005214 * A124093 A025061 A284741
Adjacent sequences: A268107 A268108 A268109 * A268111 A268112 A268113


KEYWORD

nonn,more


AUTHOR

Vincenzo Librandi, Jan 30 2016


EXTENSIONS

a(18)a(19) from Daniel Suteu, Aug 05 2019


STATUS

approved



