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 A328693 #8 Nov 03 2019 16:03:34
%S A328693 1,1,2,4,10,21,50,99,246,445,1157,2047,5326,9283,24453,41859,110486,
%T A328693 187851,494411,834975,2190965,3680541,9626773,16101703,41973942,
%U A328693 69956171,181823735,302099557,783084053,1297675587,3355767395,5548173975,14317233046,23623397329,60846853307
%N A328693 Number of double palindrome structures of length n using a maximum of four different symbols.
%C A328693 A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure.
%H A328693 Andrew Howroyd, <a href="/A328693/b328693.txt">Table of n, a(n) for n = 0..200</a>
%F A328693 a(n) = Sum_{k=1..4} A284877(n,k) for n > 0.
%F A328693 a(n) = A328690(n) + A328692(n).
%Y A328693 Cf. A284877, A328690, A328692.
%K A328693 nonn
%O A328693 0,3
%A A328693 _Andrew Howroyd_, Oct 25 2019