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%I #9 Nov 03 2019 16:03:24
%S 1,1,2,4,10,21,47,92,202,355,782,1332,2839,4733,10103,16344,34762,
%T 55761,117632,186980,392790,619805,1297727,2037180,4248151,6642921,
%U 13812710,21522622,44630687,69353037,143470907,222408044,459129802,710268189,1463532722,2259952352
%N Number of double palindrome structures of length n using a maximum of three 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="/A328692/b328692.txt">Table of n, a(n) for n = 0..200</a>
%F a(n) = Sum_{k=1..3} A284877(n,k) for n > 0.
%F a(n) = A328688(n) + A328689(n) + 1.
%e For n=4, the 10 structures are: aaaa; aaab, aaba, aabb, abaa, abab, abba, abbb; abac, abcb.
%Y Cf. A284877, A328688, A328689.
%K nonn
%O 0,3
%A _Andrew Howroyd_, Oct 25 2019
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