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A328689 Number of double palindrome structures of length n using exactly three different symbols. 4
0, 0, 0, 2, 5, 21, 42, 122, 225, 570, 990, 2321, 3913, 8827, 14480, 31802, 51425, 110928, 177270, 377722, 598437, 1264307, 1990098, 4175201, 6540605, 13653822, 21301740, 44288071, 68877929, 142736091, 221392266, 457560122, 708106245, 1460194770, 2255365152, 4641851924 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure.

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..200

EXAMPLE

For n = 4, the 2 structures are: abac, abcb.

CROSSREFS

Column 3 of A284877.

Cf. A328688, A328692.

Sequence in context: A190117 A230656 A266950 * A039777 A246167 A000941

Adjacent sequences:  A328686 A328687 A328688 * A328690 A328691 A328692

KEYWORD

nonn

AUTHOR

Andrew Howroyd, Oct 25 2019

STATUS

approved

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Last modified May 12 23:35 EDT 2021. Contains 343829 sequences. (Running on oeis4.)