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 A328688 Number of double palindrome structures of length n using exactly two different symbols. 5
 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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified May 14 06:35 EDT 2021. Contains 343879 sequences. (Running on oeis4.)