A381960 Centered heptagonal numbers which are semiprime.

22, 106, 253, 386, 841, 1198, 1618, 2101, 2458, 3046, 3473, 4166, 4411, 5461, 6623, 6931, 7246, 7897, 8926, 9647, 10018, 12811, 13238, 14113, 15947, 16423, 17893, 19951, 22121, 22681, 24403, 24991, 26797, 27413, 30598, 31921, 32593, 33958, 38221, 40447, 41966, 43513

Offset: 1

Author

Massimo Kofler, Mar 11 2025

Keywords

• nonn

Examples

			A069099(3) = 22 = 21*(3-1)/2 + 1 = 2*11.
A069099(6) = 106 = 42*(6-1)/2 + 1 = 2*53.
A069099(9) = 253 = 63*(9-1)/2 + 1 = 11*23.
		

Crossrefs

Intersection of A069099 and A001358.

Cf. A360183.

Programs

Mathematica

Select[Table[(7*n^2 - 7*n + 2)/2, {n, 1, 120}], PrimeOmega[#] == 2 &] (* Amiram Eldar, Mar 11 2025 *)

PARI

select(x->bigomega(x)==2, vector(120, n, (7*n^2-7*n+2)/2)) \\ Michel Marcus, Mar 11 2025