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 A328690 Number of double palindrome structures of length n using exactly four different symbols. 4
 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 (list; graph; refs; listen; history; text; internal format)
 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: A280717 A100837 A253576 * A019011 A036842 A267844 Adjacent sequences:  A328687 A328688 A328689 * A328691 A328692 A328693 KEYWORD nonn AUTHOR Andrew Howroyd, Oct 25 2019 STATUS approved

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Last modified May 14 07:13 EDT 2021. Contains 343879 sequences. (Running on oeis4.)