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 A307791 #10 Feb 16 2025 08:33:55
%S A307791 3,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
%N A307791 Number of palindromic heptagonal numbers of length n whose index is also palindromic.
%C A307791 Is there a nonzero term beyond a(4)?
%H A307791 Patrick De Geest, <a href="https://www.worldofnumbers.com/nobase10pg2.htm">Palindromic Squares in bases 2 to 17</a>
%H A307791 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PalindromicNumber.html">Palindromic Number</a>
%e A307791 There is only one palindromic heptagonal number of length 4 whose index is also palindromic, 44->4774. Thus, a(4)=1.
%t A307791 A054910 = {0, 1, 7, 55, 616, 3553, 4774, 60606, 848848, 4615164, 5400045, 6050506, 7165445617, 62786368726, 65331413356, 73665056637, 91120102119, 345546645543, 365139931563, 947927729749, 3646334336463, 7111015101117, 717685292586717, 19480809790808491, 615857222222758516, 1465393008003935641, 8282802468642082828, 15599378333387399551, 20316023422432061302};
%t A307791 A054971 = {0, 1, 2, 5, 16, 38, 44, 156, 583, 1359, 1470, 1556, 53537, 158476, 161656, 171657, 190914, 371778, 382173, 615769, 1207698, 1686537, 16943262, 88274141, 496329416, 765609041, 1820198063, 2497949426, 2850685772}; Table[Length[Select[A054971[[Table[Select[Range[19], IntegerLength[A054910[[#]]] == n || (n == 1 && A054910[[#]] == 0) &], {n, 19}][[n]]]], PalindromeQ[#] &]], {n, 19}]
%Y A307791 Cf. A000566, A054910, A054971, A059869, A307765, A307766.
%K A307791 nonn,base,hard,more
%O A307791 1,1
%A A307791 _Robert Price_, Apr 28 2019