A253590 Indices of the primorial numbers in A098550.

1, 2, 10, 39, 229, 2484, 31775, 532977, 10038368, 229479450

Offset: 0

Author

L. Edson Jeffery, Jan 04 2015

Keywords

• nonn
• hard
• more

Crossrefs

Cf. A002110 (primorial numbers), A098550.

Programs

Mathematica

a = {1, 2, 3}; NN = Range[4, 12000]; g = {-1}; While[g[[1]] != 0, g = Flatten[{FirstPosition[NN, v_ /; GCD[a[[-1]], v] == 1 && GCD[a[[-2]], v] > 1, 0]}]; If[g[[1]] != 0, d = NN[[g]]; a = Flatten[Append[a, d[[1]]]]; NN = Delete[NN, g[[1]]]]]; a002110[n_] := Product[Prime[j], {j, n}]; Flatten[Table[Position[a, k_ /; k == a002110[n]], {n, 0, 5}]]

Formula

A098550(a(n)) = A002110(n).

Extensions

a(0) prepended by and a(6)-a(9) from Jinyuan Wang, Jan 26 2025