A328693 - OEIS

A328693

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

1, 1, 2, 4, 10, 21, 50, 99, 246, 445, 1157, 2047, 5326, 9283, 24453, 41859, 110486, 187851, 494411, 834975, 2190965, 3680541, 9626773, 16101703, 41973942, 69956171, 181823735, 302099557, 783084053, 1297675587, 3355767395, 5548173975, 14317233046, 23623397329, 60846853307

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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..4} A284877(n,k) for n > 0.

a(n) = A328690(n) + A328692(n).

CROSSREFS

Cf. A284877, A328690, A328692.