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A056503 Number of periodic palindromic structures of length n using a maximum of two different symbols. 10
1, 2, 2, 4, 4, 7, 8, 14, 16, 26, 32, 51, 64, 100, 128, 198, 256, 392, 512, 778, 1024, 1552, 2048, 3091, 4096, 6176, 8192, 12324, 16384, 24640, 32768, 49222, 65536, 98432, 131072, 196744, 262144, 393472, 524288, 786698, 1048576, 1573376, 2097152, 3146256, 4194304 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure.

A periodic palindrome is just a necklace that is equivalent to its reverse. The number of binary periodic palindromes of length n is given by A164090(n). A binary periodic palindrome can only be equivalent to its complement when there are an equal number of 0's and 1's. - Andrew Howroyd, Sep 29 2017

Number of cyclic compositions (necklaces of positive integers) summing to n that can be rotated to form a palindrome. - Gus Wiseman, Sep 16 2018

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

Andrew Howroyd, Table of n, a(n) for n = 1..200

FORMULA

a(2n+1) = A164090(2n+1)/2 = 2^n, a(2n) = (A164090(2n) + A045674(n))/2. - Andrew Howroyd, Sep 29 2017

EXAMPLE

From Gus Wiseman, Sep 16 2018: (Start)

The sequence of palindromic cyclic compositions begins:

  (1)  (2)   (3)    (4)     (5)      (6)       (7)

       (11)  (111)  (22)    (113)    (33)      (115)

                    (112)   (122)    (114)     (133)

                    (1111)  (11111)  (222)     (223)

                                     (1122)    (11113)

                                     (11112)   (11212)

                                     (111111)  (11122)

                                               (1111111)

(End)

MATHEMATICA

(* b = A164090, c = A045674 *)

b[n_] := (1/4)*(7 - (-1)^n)*2^((1/4)*(2*n + (-1)^n - 1));

c[0] = 1; c[n_] := c[n] = If[EvenQ[n], 2^(n/2-1) + c[n/2], 2^((n-1)/2)];

a[n_?OddQ] := b[n]/2; a[n_?EvenQ] := (1/2)*(b[n] + c[n/2]);

Array[a, 45] (* Jean-Fran├žois Alcover, Oct 08 2017, after Andrew Howroyd *)

Table[Length[Select[Join@@Permutations/@IntegerPartitions[n], Function[q, And[Array[OrderedQ[{q, RotateRight[q, #]}]&, Length[q]-1, 1, And], Array[SameQ[RotateRight[q, #], Reverse[RotateRight[q, #]]]&, Length[q], 1, Or]]]]], {n, 15}] (* Gus Wiseman, Sep 16 2018 *)

CROSSREFS

Row sums of A179181.

Cf. A016116, A045674, A056508, A164090, A285012.

Cf. A000740, A000837, A008965, A025065, A059966, A242414, A296302, A317085, A317086, A317087, A318731.

Sequence in context: A222738 A005308 A151532 * A256217 A055636 A206559

Adjacent sequences:  A056500 A056501 A056502 * A056504 A056505 A056506

KEYWORD

nonn

AUTHOR

Marks R. Nester

EXTENSIONS

a(17)-a(45) from Andrew Howroyd, Apr 07 2017

STATUS

approved

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Last modified July 23 14:44 EDT 2021. Contains 346259 sequences. (Running on oeis4.)