A318728 Number of cyclic compositions (necklaces of positive integers) summing to n that have only one part or whose adjacent parts (including the last with first) are coprime.

1, 2, 3, 4, 6, 9, 13, 22, 34, 58, 95, 168, 280, 492, 853, 1508, 2648, 4715, 8350, 14924, 26643, 47794, 85779, 154475, 278323, 502716, 908913, 1646206, 2984547, 5418653, 9847190, 17916001, 32625618, 59470540, 108493150, 198094483, 361965239, 661891580, 1211162271

Offset: 1

Author

Gus Wiseman, Sep 02 2018

Keywords

• nonn

Examples

			The a(7) = 13 cyclic compositions with adjacent parts coprime:
  7,
  16, 25, 34,
  115,
  1114, 1213, 1132, 1123,
  11113, 11212,
  111112,
  1111111.
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..100

Crossrefs

Cf. A000740, A000837, A008965, A059966, A100953, A167606, A296302, A318726, A318727, A328597.

Programs

Mathematica

neckQ[q_]:=Array[OrderedQ[{q,RotateRight[q,#]}]&,Length[q]-1,1,And];
Table[Length[Select[Join@@Permutations/@IntegerPartitions[n],Or[Length[#]==1,neckQ[#]&&And@@CoprimeQ@@@Partition[#,2,1,1]]&]],{n,20}]

PARI

b(n, q, pred)={my(M=matrix(n, n)); for(k=1, n, M[k, k]=pred(q, k); for(i=1, k-1, M[i, k]=sum(j=1, k-i, if(pred(j, i), M[j, k-i], 0)))); M[q,]}
seq(n)={my(v=sum(k=1, n, k*b(n, k, (i,j)->gcd(i,j)==1))); vector(n, n, (n > 1) + sumdiv(n, d, eulerphi(d)*v[n/d])/n)} \\ Andrew Howroyd, Oct 27 2019

Formula

a(n) = A328597(n) + 1 for n > 1. - Andrew Howroyd, Oct 27 2019

Extensions

Terms a(21) and beyond from Andrew Howroyd, Sep 08 2018

Name corrected by Gus Wiseman, Nov 04 2019