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 A328688 #9 Nov 03 2019 16:03:02 %S A328688 0,1,3,7,15,25,49,79,129,211,341,517,819,1275,1863,2959,4335,6703, %T A328688 9709,15067,21367,33419,47081,72949,102315,158887,220881,342615, %U A328688 475107,734815,1015777,1569679,2161943,3337951,4587199,7069747,9699291,14932443,20445519,31441339 %N A328688 Number of double palindrome structures of length n using exactly two different symbols. %C A328688 A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure. %H A328688 Andrew Howroyd, <a href="/A328688/b328688.txt">Table of n, a(n) for n = 1..1000</a> %e A328688 For n = 4, the 7 structures are: aaab, aaba, aabb, abaa, abab, abba, abbb. %Y A328688 Column 2 of A284877. %Y A328688 Cf. A328689, A328690. %K A328688 nonn %O A328688 1,3 %A A328688 _Andrew Howroyd_, Oct 25 2019