A382132 Centered pentagonal numbers which are semiprimes.

6, 51, 106, 141, 226, 391, 526, 681, 766, 951, 1501, 1891, 2031, 2326, 2481, 2641, 3151, 3901, 4101, 4306, 6631, 6891, 7981, 8266, 8851, 10081, 10401, 11391, 13141, 14631, 15406, 16201, 20931, 22801, 23281, 24751, 27301, 27826, 28891, 29431, 30526, 32206, 33351, 35701, 36301, 38131, 38751

Offset: 1

Author

Massimo Kofler, Mar 17 2025

Keywords

• nonn

Examples

			A005891(1) = 6 = (5*1^2 + 5*1 + 2)/2 = 2*3.
A005891(4) = 51 = (5*4^2 + 5*4 + 2)/2 = 3*17.
A005891(6) = 106 = (5*6^2 + 5*6 + 2)/2 = 2*53.
		

Crossrefs

Intersection of A001358 and A005891.

Cf. A364610.

Programs

Mathematica

Select[Table[(5*n^2 + 5*n + 2)/2, {n, 1, 125}], PrimeOmega[#] == 2 &] (* Amiram Eldar, Mar 17 2025 *)