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 A328689 #9 Nov 03 2019 16:03:10 %S A328689 0,0,0,2,5,21,42,122,225,570,990,2321,3913,8827,14480,31802,51425, %T A328689 110928,177270,377722,598437,1264307,1990098,4175201,6540605,13653822, %U A328689 21301740,44288071,68877929,142736091,221392266,457560122,708106245,1460194770,2255365152,4641851924 %N A328689 Number of double palindrome structures of length n using exactly three different symbols. %C A328689 A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure. %H A328689 Andrew Howroyd, <a href="/A328689/b328689.txt">Table of n, a(n) for n = 1..200</a> %e A328689 For n = 4, the 2 structures are: abac, abcb. %Y A328689 Column 3 of A284877. %Y A328689 Cf. A328688, A328692. %K A328689 nonn %O A328689 1,4 %A A328689 _Andrew Howroyd_, Oct 25 2019