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 A328690 #8 Nov 03 2019 16:03:19 %S A328690 0,0,0,0,0,3,7,44,90,375,715,2487,4550,14350,25515,75724,132090, %T A328690 376779,647995,1798175,3060736,8329046,14064523,37725791,63313250, %U A328690 168011025,280576935,738453366,1228322550,3212296488,5325765931,13858103244,22913129140,59383320585 %N A328690 Number of double palindrome structures of length n using exactly four different symbols. %C A328690 A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure. %H A328690 Andrew Howroyd, <a href="/A328690/b328690.txt">Table of n, a(n) for n = 1..200</a> %Y A328690 Column 4 of A284877. %Y A328690 Cf. A328688, A328693. %K A328690 nonn %O A328690 1,6 %A A328690 _Andrew Howroyd_, Oct 25 2019