A335489 Number of strict permutations of the prime indices of n.

1, 1, 1, 0, 1, 2, 1, 0, 0, 2, 1, 0, 1, 2, 2, 0, 1, 0, 1, 0, 2, 2, 1, 0, 0, 2, 0, 0, 1, 6, 1, 0, 2, 2, 2, 0, 1, 2, 2, 0, 1, 6, 1, 0, 0, 2, 1, 0, 0, 0, 2, 0, 1, 0, 2, 0, 2, 2, 1, 0, 1, 2, 0, 0, 2, 6, 1, 0, 2, 6, 1, 0, 1, 2, 0, 0, 2, 6, 1, 0, 0, 2, 1, 0, 2, 2, 2

Offset: 1

Author

Gus Wiseman, Jun 19 2020

Keywords

• nonn

Comments

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

Also the number of (1,1)-avoiding permutations of the prime indices of n.

Links

• Wikipedia, Permutation pattern
• Gus Wiseman, Sequences counting and ranking compositions by the patterns they match or avoid.

Crossrefs

Positions of first appearances are A002110 with 2 replaced by 4.

Permutations of prime indices are counted by A008480.

The contiguous version is A335451.

Anti-run permutations of prime indices are counted by A335452.

(1,1,1)-avoiding permutations of prime indices are counted by A335511.

Cf. A056239, A056986, A106356, A112798, A238279, A281188, A333221, A335456, A335460, A335462, A335465.

Programs

Mathematica

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Table[Length[Select[Permutations[primeMS[n]],!MatchQ[#,{_,x_,_,x_,_}]&]],{n,100}]

Formula

If n is squarefree, a(n) = A001221(n)!; otherwise a(n) = 0.

a(n != 4) = A281188(n); a(4) = 0.