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%I #9 Nov 03 2019 16:03:10
%S 0,0,0,2,5,21,42,122,225,570,990,2321,3913,8827,14480,31802,51425,
%T 110928,177270,377722,598437,1264307,1990098,4175201,6540605,13653822,
%U 21301740,44288071,68877929,142736091,221392266,457560122,708106245,1460194770,2255365152,4641851924
%N Number of double palindrome structures of length n using exactly 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="/A328689/b328689.txt">Table of n, a(n) for n = 1..200</a>
%e For n = 4, the 2 structures are: abac, abcb.
%Y Column 3 of A284877.
%Y Cf. A328688, A328692.
%K nonn
%O 1,4
%A _Andrew Howroyd_, Oct 25 2019