A328689 Number of double palindrome structures of length n using exactly three different symbols.

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

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 A359725

Adjacent sequences: A328686 A328687 A328688 * A328690 A328691 A328692

Keyword

nonn

Author

Andrew Howroyd, Oct 25 2019

Status

approved