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 A307808 #10 Feb 16 2025 08:33:55
%S A307808 3,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
%N A307808 Number of palindromic nonagonal numbers of length n whose index is also palindromic.
%C A307808 Is there a nonzero term beyond a(4)?
%H A307808 Patrick De Geest, <a href="https://www.worldofnumbers.com/nobase10pg2.htm">Palindromic Squares in bases 2 to 17</a>
%H A307808 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PalindromicNumber.html">Palindromic Number</a>
%e A307808 There is only one palindromic nonagonal number of length 4 whose index is also palindromic, 44->6666. Thus, a(4)=1.
%t A307808 A082723 = {0, 1, 9, 111, 474, 969, 6666, 18981, 67276, 4411144, 6964696, 15444451, 57966975, 448707844, 460595064, 579696975, 931929139, 994040499, 1227667221, 9698998969, 61556965516, 664248842466, 699030030996, 99451743334715499, 428987160061789824, 950178723327871059, 1757445628265447571, 4404972454542794044, 9433971680861793349, 499583536595635385994, 1637992008558002997361, 19874891310701319847891};
%t A307808 A055560 = {0, 1, 2, 6, 12, 17, 44, 74, 139, 1123, 1411, 2101, 4070, 11323, 11472, 12870, 16318, 16853, 18729, 52642, 132619, 435644, 446904, 168566853, 350096787, 521037077, 708609429, 1121857192, 1641773578, 11947307367, 21633254881, 75356090494};
%t A307808 Table[Length[Select[A055560[[Table[Select[Range[22], IntegerLength[A082723[[#]]] == n || (n == 1 && A082723[[#]] == 0) &], {n, 22}][[n]]]], PalindromeQ[#] &]], {n, 22}]
%Y A307808 Cf. A001106, A055560, A082722, A082733, A307801, A307802.
%K A307808 nonn,base,hard,more
%O A307808 1,1
%A A307808 _Robert Price_, Apr 29 2019