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 A328692 Number of double palindrome structures of length n using a maximum of three different symbols. 4
 1, 1, 2, 4, 10, 21, 47, 92, 202, 355, 782, 1332, 2839, 4733, 10103, 16344, 34762, 55761, 117632, 186980, 392790, 619805, 1297727, 2037180, 4248151, 6642921, 13812710, 21522622, 44630687, 69353037, 143470907, 222408044, 459129802, 710268189, 1463532722, 2259952352 (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..3} A284877(n,k) for n > 0. a(n) = A328688(n) + A328689(n) + 1. EXAMPLE For n=4, the 10 structures are: aaaa; aaab, aaba, aabb, abaa, abab, abba, abbb; abac, abcb. CROSSREFS Cf. A284877, A328688, A328689. Sequence in context: A128461 A011956 A018003 * A255711 A204804 A328693 Adjacent sequences:  A328689 A328690 A328691 * A328693 A328694 A328695 KEYWORD nonn AUTHOR Andrew Howroyd, Oct 25 2019 STATUS approved

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Last modified May 16 14:39 EDT 2021. Contains 343949 sequences. (Running on oeis4.)