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 A328693 Number of double palindrome structures of length n using a maximum of four different symbols. 3
 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 (list; graph; refs; listen; history; text; internal format)
 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. Sequence in context: A328692 A255711 A204804 * A165136 A165137 A065023 Adjacent sequences:  A328690 A328691 A328692 * A328694 A328695 A328696 KEYWORD nonn AUTHOR Andrew Howroyd, Oct 25 2019 STATUS approved

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Last modified May 15 13:52 EDT 2021. Contains 343920 sequences. (Running on oeis4.)