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