A328688 Number of double palindrome structures of length n using exactly two different symbols.

0, 1, 3, 7, 15, 25, 49, 79, 129, 211, 341, 517, 819, 1275, 1863, 2959, 4335, 6703, 9709, 15067, 21367, 33419, 47081, 72949, 102315, 158887, 220881, 342615, 475107, 734815, 1015777, 1569679, 2161943, 3337951, 4587199, 7069747, 9699291, 14932443, 20445519, 31441339

Offset

1,3

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 = 1..1000

Example

For n = 4, the 7 structures are: aaab, aaba, aabb, abaa, abab, abba, abbb.

Crossrefs

Column 2 of A284877.

Cf. A328689, A328690.

Sequence in context: A175510 A144643 A034757 * A291651 A078869 A011890

Adjacent sequences: A328685 A328686 A328687 * A328689 A328690 A328691

Keyword

nonn

Author

Andrew Howroyd, Oct 25 2019

Status

approved