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 A055560 #10 Aug 11 2024 14:41:33 %S A055560 1,2,6,12,17,44,74,139,1123,1411,2101,4070,11323,11472,12870,16318, %T A055560 16853,18729,52642,132619,435644,446904,168566853,350096787,521037077, %U A055560 708609429,1121857192,1641773578,11947307367,21633254881,75356090494 %N A055560 Base numbers of 9-gonal palindromic numbers. %H A055560 P. De Geest, <a href="https://www.worldofnumbers.com/nona.htm">Palindromic nonagonals</a> %F A055560 9-gonal numbers are of the form (n(7n-5))/2. %t A055560 Select[Range[0, 10^5], PalindromeQ[PolygonalNumber[9, #]] &] (* _Robert Price_, Apr 29 2019 *) %Y A055560 Cf. A001106, A082723. %K A055560 nonn,base %O A055560 1,2 %A A055560 _J. Lowell_, Jul 10 2000 %E A055560 Edited and extended by _Patrick De Geest_, Apr 13 2003.