A180249 - OEIS

A180249

a(n) is the total number of k-reverses of n.

1, 2, 4, 8, 16, 26, 50, 80, 130, 212, 342, 518, 820, 1276, 1864, 2960, 4336, 6704, 9710, 15068, 21368, 33420, 47082, 72950, 102316, 158888, 220882, 342616, 475108, 734816, 1015778, 1569680, 2161944, 3337952, 4587200, 7069748, 9699292, 14932444, 20445520

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OFFSET

1,2

COMMENTS

See sequence A180171 for the definition of a k-reverse of n.

Briefly, a k-reverse of n is a k-composition of n whose reverse is cyclically equivalent to itself.

This sequence is the total number of k-reverses of n for k=1,2,...,n.

It is the row sums of the 'R(n,k)' triangle from sequence A180171.

For example a(6)=26 because there are 26 k-reverses of n=6 for k=1,2,3,4,5, or 6.

They are, in cyclically equivalent, classes: {6}, {15,51}, {24,42},{33},{114,411,141},{222} {1113,3111,1311,1131}, {1122,2112,2211,1221}, {1212,2121}, {11112,21111,12111,11211,11121}, {111111}.

REFERENCES

John P. McSorley: Counting k-compositions with palindromic and related structures. Preprint, 2010.

LINKS

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

FORMULA

a(n) = Sum_{d|n} d*A056493(d)/2. - Andrew Howroyd, Oct 07 2017

From Petros Hadjicostas, Oct 15 2017: (Start)

a(n) = (n/2)*Sum_{d|n} (phi^(-1)(d)/d)*b(n/d), where phi^(-1)(n) = A023900(n) is the Dirichlet inverse of the Euler totient function and b(n) = A029744(n+1) (= 3*2^((n/2)-1), if n is even, and = 2^((n+1)/2), if n is odd).

G.f.: Sum_{n>=1} phi^(-1)(n)*g(x^n), where phi^(-1)(n) = A023900(n) and g(x) = x*(x+1)*(2*x+1)/(1-2*x^2)^2.

(End)

MATHEMATICA

f[n_Integer] := Block[{c = 0, k = 1, ip = IntegerPartitions@ n, lmt = 1 + PartitionsP@ n, ipk}, While[k < lmt, c += g[ ip[[k]]]; k++ ]; c]; g[lst_List] := Block[{c = 0, len = Length@ lst, per = Permutations@ lst}, While[ Length@ per > 0, rl = Union[ RotateLeft[ per[[1]], # ] & /@ Range@ len]; If[ MemberQ[rl, Reverse@ per[[1]]], c += Length@ rl]; per = Complement[ per, rl]]; c]; Array[f, 24] (* Robert G. Wilson v, Aug 25 2010 *)

b[n_] := Sum[MoebiusMu[n/d] * If[OddQ[d], 2, 3] * 2^Quotient[d-1, 2], {d, Divisors[n]}]; a[n_] := Sum[d*b[d], {d, Divisors[n]}] / 2; Array[a, 39] (* Jean-François Alcover, Nov 04 2017, after Andrew Howroyd *)

PROG

(PARI) \\ here b(n) is A056493

b(n) = sumdiv(n, d, moebius(n/d) * if(d%2, 2, 3) * 2^((d-1)\2));

a(n) = sumdiv(n, d, d*b(d)) / 2; \\ Andrew Howroyd, Oct 07 2017

CROSSREFS

If we ask for the number of cyclically equivalent classes we get sequence A052955.

For example the 6th term of A052955 is 11, corresponding to the 11 classes in the example above.

Row sums of A180171.

Cf. A056493, A180322.

Sequence in context: A354146 A354255 A319385 * A060957 A322326 A018826

Adjacent sequences: A180246 A180247 A180248 * A180250 A180251 A180252

KEYWORD