A387136 Number of ways to choose a sequence of distinct prime factors, one of each prime index of 2n - 1.

1, 1, 1, 1, 0, 1, 2, 1, 1, 1, 0, 1, 0, 0, 2, 1, 1, 1, 2, 1, 1, 2, 0, 2, 0, 1, 1, 1, 0, 1, 2, 0, 1, 1, 1, 2, 2, 0, 1, 2, 0, 1, 1, 1, 2, 1, 1, 1, 1, 0, 2, 1, 0, 2, 1, 1, 3, 0, 0, 1, 0, 1, 0, 1, 1, 1, 0, 0, 2, 2, 2, 2, 2, 0, 2, 2, 0, 1, 1, 0, 1, 2, 1, 2, 2, 0, 2

Offset: 1

Author

Gus Wiseman, Aug 30 2025

Keywords

• nonn
• new

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.

Examples

			The prime indices of 4537 are {6,70}, with choices (2,5), (2,7), (3,2), (3,5), (3,7). Since 4537 = 2 * 2269 - 1, we have a(2269) = 5.
		

Crossrefs

Here we use the version with alternating zeros (put n instead of 2n - 1 in the name).

Twice partitions of this type are counted by A296122.

Positions of zero are A355529, complement A368100.

For divisors instead of prime factors we have A355739.

Allowing repeated choices gives A355741.

For partitions instead of prime factors we have A387110.

For initial intervals instead of prime factors we have A387111.

For strict partitions instead of prime factors we have A387115, disjoint case A383706.

For constant partitions instead of prime factors we have A387120.

A000041 counts integer partitions, strict A000009.

A003963 multiplies together prime indices.

A112798 lists prime indices, row sums A056239 or A066328, lengths A001222.

A120383 lists numbers divisible by all of their prime indices.

A289509 lists numbers with relatively prime prime indices.

Cf. A261049, A276078, A276079, A299200, A335433, A335448, A355731, A355745, A367771, A387133, A387135, A387327.

Programs

Mathematica

prix[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Table[Length[Select[Tuples[If[#==1,{},First/@FactorInteger[#]]&/@prix[2n-1]],UnsameQ@@#&]],{n,100}]