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

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.

Links

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

Crossrefs

Column 4 of A284877.

Cf. A328688, A328693.