A363837 - OEIS

%I #17 Feb 25 2024 02:16:20

%S 6,8,10,14,26,29,34,37,38,41,47,49,53,62,67,71,73,103,107,109,122,139,

%T 151,179,223,229,251,254,269,277,311,349,353,433,457,487,503,599,601,

%U 613,619,643,739,757,827,839,1031,1061,1117

%N Numbers k such that k-th Jacobsthal number A001045(k) is a semiprime.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/JacobsthalNumber.html">Jacobsthal Number</a>.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Semiprime.html">Semiprime</a>.

%e 10 is a term because Jacobsthal(10) = A001045(10) = 341 = 11*31 is a semiprime.

%o (PARI) isok(k) = bigomega((2^k - (-1)^k)/3) == 2; \\ _Michel Marcus_, Oct 19 2023

%Y Cf. A001045, A001358, A277356 (the actual semiprimes), A250292, A085726, A072381, A101757, A286567, A271314.

%K nonn,more

%O 1,1

%A _Sean A. Irvine_, Oct 19 2023

%E a(47)-a(49) from _Amiram Eldar_, Feb 25 2024