A328692 Number of double palindrome structures of length n using a maximum of three different symbols.

1, 1, 2, 4, 10, 21, 47, 92, 202, 355, 782, 1332, 2839, 4733, 10103, 16344, 34762, 55761, 117632, 186980, 392790, 619805, 1297727, 2037180, 4248151, 6642921, 13812710, 21522622, 44630687, 69353037, 143470907, 222408044, 459129802, 710268189, 1463532722, 2259952352

Offset: 0

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 10 structures are: aaaa; aaab, aaba, aabb, abaa, abab, abba, abbb; abac, abcb.
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..200

Crossrefs

Cf. A284877, A328688, A328689.

Formula

a(n) = Sum_{k=1..3} A284877(n,k) for n > 0.

a(n) = A328688(n) + A328689(n) + 1.