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

0,3

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 = 0..200

Formula

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

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

Example

For n=4, the 10 structures are: aaaa; aaab, aaba, aabb, abaa, abab, abba, abbb; abac, abcb.

Crossrefs

Cf. A284877, A328688, A328689.

Sequence in context: A011956 A371358 A018003 * A255711 A204804 A328693

Adjacent sequences: A328689 A328690 A328691 * A328693 A328694 A328695

Keyword

nonn

Author

Andrew Howroyd, Oct 25 2019

Status

approved