A328690 Number of double palindrome structures of length n using exactly four different symbols.

0, 0, 0, 0, 0, 3, 7, 44, 90, 375, 715, 2487, 4550, 14350, 25515, 75724, 132090, 376779, 647995, 1798175, 3060736, 8329046, 14064523, 37725791, 63313250, 168011025, 280576935, 738453366, 1228322550, 3212296488, 5325765931, 13858103244, 22913129140, 59383320585

Offset

1,6

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

Crossrefs

Column 4 of A284877.

Cf. A328688, A328693.

Sequence in context: A100837 A253576 A354428 * A019011 A036842 A267844

Adjacent sequences: A328687 A328688 A328689 * A328691 A328692 A328693

Keyword

nonn

Author

Andrew Howroyd, Oct 25 2019

Status

approved