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 A115657 #13 Jan 14 2024 13:32:28 %S A115657 0,1,5,22,210,287,782,1001,2882,7740,15251,77180,565187,720027,781565, %T A115657 7081807,7451547,26811862,54177145,206489067,246214610,760984602, %U A115657 1050660501,1085885801,1528888251,2642326276,2911771192,6726232462,24375132126,62123157342 %N A115657 Both k and the reverse of k are pentagonal numbers (A000326). %e A115657 565187=P(614) and 781565=P(722), where P(k) = k*(3*k-1)/2 is the k-th pentagonal number. %o A115657 (PARI) lista(nn) = for (n=0, nn, my(P = n*(3*n-1)/2); if (ispolygonal(fromdigits(Vecrev(digits(P))), 5), print1(P, ", "));); \\ _Michel Marcus_, May 22 2022 %Y A115657 Cf. A000326, A061455, A061457. %K A115657 base,nonn %O A115657 1,3 %A A115657 _Giovanni Resta_, Jan 28 2006 %E A115657 0 and additional terms from _Jon E. Schoenfield_, May 22 2022