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 A056508 Number of periodic palindromic structures of length n using exactly two different symbols. 3
 0, 1, 1, 3, 3, 6, 7, 13, 15, 25, 31, 50, 63, 99, 127, 197, 255, 391, 511, 777, 1023, 1551, 2047, 3090, 4095, 6175, 8191, 12323, 16383, 24639, 32767, 49221, 65535, 98431, 131071, 196743, 262143, 393471, 524287, 786697, 1048575, 1573375, 2097151, 3146255, 4194303 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure. For odd n, a palindrome cannot be the complement of itself, so a(n) is given by A284855(n,2)/2 - 1. - Andrew Howroyd, Apr 08 2017 REFERENCES M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2] LINKS Giovanni Resta, Table of n, a(n) for n = 1..1000 FORMULA a(n) = A056503(n) - 1. a(2n + 1) = 2^n - 1. - Andrew Howroyd, Apr 07 2017 EXAMPLE From Andrew Howroyd, Apr 07 2017: (Start) Example for n=6: Periodic symmetry means results are either in the form abccba or abcdcb. There are 3 binary words in the form abccba that start with 0 and contain a 1 which are 001100, 010010, 011110. Of these, 011110 is equivalent to 001100 after rotation. There are 7 binary words in the form abcdcb that start with 0 and contain a 1 which are 000100, 001010, 001110, 010001, 010101, 011011, 011111. Of these, 011111 is equivalent to 000100, 010001 is equivalent to 001010 and 011011 is equivalent to 010010 from the first set. There are therefore a total of 7 + 3 - 4 = 6 equivalence classes so a(6) = 6. (End) MATHEMATICA (* b = A164090, c = A045674 *) b[n_] := (1/4)*(7 - (-1)^n)*2^((1/4)*(2*n + (-1)^n - 1)); c = 1; c[n_] := c[n] = If[EvenQ[n], 2^(n/2 - 1) + c[n/2], 2^((n - 1)/2)]; a[n_] := If[OddQ[n], b[n]/2, (1/2)*(b[n] + c[n/2])] - 1; Array[a, 45] (* Jean-François Alcover, Jun 29 2018, after Andrew Howroyd *) CROSSREFS Column 2 of A285012. Cf. A052551. Sequence in context: A325834 A241832 A027187 * A050065 A298732 A078477 Adjacent sequences:  A056505 A056506 A056507 * A056509 A056510 A056511 KEYWORD nonn AUTHOR EXTENSIONS a(17)-a(45) from Andrew Howroyd, Apr 07 2017 STATUS approved

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Last modified November 12 19:41 EST 2019. Contains 329078 sequences. (Running on oeis4.)