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

Author

Andrew Howroyd, Oct 25 2019

Keywords

• nonn

Comments

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

Examples

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

Links

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

Crossrefs

Column 3 of A284877.

Cf. A328688, A328692.