This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A381043 #13 Apr 18 2025 21:20:02
%S A381043 6,51,106,141,226,391,526,681,766,951,1501,1891,2031,2326,2481,2641,
%T A381043 3151,3901,4101,4306,6631,6891,7981,8266,8851,10081,10401,11391,13141,
%U A381043 14631,15406,16201,20931,23281,24751,27301,27826,28891,29431,30526,32206,33351,35701,36301,38131,38751,41926
%N A381043 Centered pentagonal numbers which are squarefree semiprimes.
%C A381043 Numbers such as 22801=151^2 and 1666681=1291^2 are in A382132 but not here.
%e A381043 A005891(1) = 6 = (5*1^2 + 5*1 + 2)/2 = 2*3.
%e A381043 A005891(4) = 51 = (5*4^2 + 5*4 + 2)/2 = 3*17.
%e A381043 A005891(6) = 106 = (5*6^2 + 5*6 + 2)/2 = 2*53.
%t A381043 Select[Table[5*n*(n + 1)/2 + 1, {n, 0, 150}], FactorInteger[#][[;; , 2]] == {1, 1} &] (* _Amiram Eldar_, Apr 14 2025 *)
%Y A381043 Intersection of A006881 and A005891.
%Y A381043 Cf. A145838, A382132, A364610.
%K A381043 nonn
%O A381043 1,1
%A A381043 _Massimo Kofler_, Apr 14 2025