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A328692
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Number of double palindrome structures of length n using a maximum of three different symbols.
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4
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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
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OFFSET
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0,3
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COMMENTS
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A double palindrome is the concatenation of two palindromes. Permuting the symbols will not change the structure.
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LINKS
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FORMULA
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a(n) = Sum_{k=1..3} A284877(n,k) for n > 0.
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EXAMPLE
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For n=4, the 10 structures are: aaaa; aaab, aaba, aabb, abaa, abab, abba, abbb; abac, abcb.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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