A113527 Semiprimes in A056108.

15, 115, 155, 201, 253, 445, 785, 1345, 2215, 3503, 3711, 4145, 4841, 5853, 6395, 7855, 9131, 12353, 13535, 14353, 16503, 18331, 19281, 20255, 20751, 21253, 21761, 23853, 24935, 26603, 29503, 30101

Offset

1,1

Comments

Intersection of A056108 and A001358.

Links

Michael De Vlieger, Table of n, a(n) for n = 1..10000

Formula

{a(n)} = {3*n^2 + n + 1 iff semiprime}. {a(n)} = A056108 INTERSECT A001358.

Example

a(1) = 15 because A056108(2) = 15 = 3 * 5 is semiprime.
a(2) = 115 because A056108(6) = 115 = 5 * 23 is semiprime.
a(32) = 30101 because A056108(100) = 30101 = 31 * 971 is semiprime.

Mathematica

Select[Array[3 #^2 + # + 1 &, 100], PrimeOmega[#] == 2 &] (* Michael De Vlieger, Mar 17 2021 *)

Crossrefs

Cf. A001358, A056108.

Cf. A113519, A113524, A113525, A113528, A113530.

Sequence in context: A244872 A293875 A044347 * A044728 A091347 A125352

Adjacent sequences: A113524 A113525 A113526 * A113528 A113529 A113530

Keyword

easy,nonn

Author

Jonathan Vos Post, Jan 12 2006

Status

approved