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A277356
Jacobsthal numbers which are semiprimes.
2
21, 85, 341, 5461, 22369621, 178956971, 5726623061, 45812984491, 91625968981, 733007751851, 46912496118443, 187649984473771, 3002399751580331, 1537228672809129301, 49191317529892137643, 787061080478274202283, 3148244321913096809131
OFFSET
1,1
COMMENTS
Semiprimes of the form (2^k - (-1)^k)/3.
LINKS
Eric Weisstein's World of Mathematics, Jacobsthal Number.
Eric Weisstein's World of Mathematics, Semiprime.
FORMULA
a(n) = A001045(A363837(n)). - Amiram Eldar, Feb 25 2024
EXAMPLE
a(1) = 21 because 21 = 3*7 = (2^6 - (-1)^6)/3, so 21 is semiprime as well as a Jacobsthal number;
a(2) = 85 because 85 = 5*17 = (2^8 - (-1)^8)/3;
a(3) = 341 because 341 = 11*31 = (2^10 - (-1)^10)/3, etc.
MATHEMATICA
Select[Table[(2^k - (-1)^k)/3, {k, 100}], PrimeOmega[#1] == 2 & ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Oct 10 2016
STATUS
approved