A381043 Centered pentagonal numbers which are squarefree 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, 23281, 24751, 27301, 27826, 28891, 29431, 30526, 32206, 33351, 35701, 36301, 38131, 38751, 41926
Offset: 1
Keywords
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.
Programs
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Mathematica
Select[Table[5*n*(n + 1)/2 + 1, {n, 0, 150}], FactorInteger[#][[;; , 2]] == {1, 1} &] (* Amiram Eldar, Apr 14 2025 *)
Comments