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 A385146 #18 Jul 23 2025 16:12:37 %S A385146 0,5,11310,55089888425,121583358792810,592213938425857857485, %T A385146 1307015893771450525626150,6366274445128720143976790614385, %U A385146 14050364815815600018103892686892130,68437177312018083317895951729386272287125,151040819318475127652012254231608319757368350 %N A385146 Pentagonal numbers that are five times another pentagonal number. %F A385146 a(n) = 5 * A385152(n). %e A385146 P = 11310 is a term since P and P/5 = 2262 are both pentagonal numbers. %o A385146 (PARI) is_a000326(n) = my(s); n==0 || (issquare (24*n+1, &s) && s%6==5); %o A385146 for(n=0,oo,p=n*(3*n-1)/2; if(p%5==0 && is_a000326(p/5),print1(p,", "))); \\ _Joerg Arndt_, Jun 19 2025 %Y A385146 Cf. A000326, A076140, A385152. %K A385146 nonn %O A385146 1,2 %A A385146 _Kelvin Voskuijl_, Jun 19 2025