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

1,1

Links

Table of n, a(n) for n=1..47.

Example

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.

Mathematica

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

Crossrefs

Intersection of A001358 and A005891.

Cf. A364610.

Sequence in context: A173082 A113653 A372336 * A381043 A133395 A341424

Adjacent sequences: A382129 A382130 A382131 * A382133 A382134 A382135

Keyword

nonn

Author

Massimo Kofler, Mar 17 2025

Status

approved