A363837 Numbers k such that k-th Jacobsthal number A001045(k) is a semiprime.

6, 8, 10, 14, 26, 29, 34, 37, 38, 41, 47, 49, 53, 62, 67, 71, 73, 103, 107, 109, 122, 139, 151, 179, 223, 229, 251, 254, 269, 277, 311, 349, 353, 433, 457, 487, 503, 599, 601, 613, 619, 643, 739, 757, 827, 839, 1031, 1061, 1117

Offset

1,1

Links

Table of n, a(n) for n=1..49.

Eric Weisstein's World of Mathematics, Jacobsthal Number.

Eric Weisstein's World of Mathematics, Semiprime.

Example

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

Prog

(PARI) isok(k) = bigomega((2^k - (-1)^k)/3) == 2; \\ Michel Marcus, Oct 19 2023

Crossrefs

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

Sequence in context: A065858 A243537 A073582 * A048886 A349477 A161545

Adjacent sequences: A363834 A363835 A363836 * A363838 A363839 A363840

Keyword

nonn,more

Author

Sean A. Irvine, Oct 19 2023

Extensions

a(47)-a(49) from Amiram Eldar, Feb 25 2024

Status

approved