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
Examples
10 is a term because Jacobsthal(10) = A001045(10) = 341 = 11*31 is a semiprime.
Links
- Eric Weisstein's World of Mathematics, Jacobsthal Number.
- Eric Weisstein's World of Mathematics, Semiprime.
Crossrefs
Programs
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PARI
isok(k) = bigomega((2^k - (-1)^k)/3) == 2; \\ Michel Marcus, Oct 19 2023
Extensions
a(47)-a(49) from Amiram Eldar, Feb 25 2024