A126286 a(1) = 2, a(n) = n * LeastPrimeFactor(n+1) / LeastPrimeFactor(n).

2, 3, 2, 10, 2, 21, 2, 12, 6, 55, 2, 78, 2, 21, 10, 136, 2, 171, 2, 30, 14, 253, 2, 60, 10, 39, 18, 406, 2, 465, 2, 48, 22, 85, 14, 666, 2, 57, 26, 820, 2, 903, 2, 66, 30, 1081, 2, 168, 14, 75, 34, 1378, 2, 135, 22, 84, 38, 1711, 2, 1830, 2, 93, 42, 160, 26, 2211, 2, 102, 46

Offset: 1

Author

Lior Manor, Dec 25 2006

Keywords

• easy
• nonn

Examples

			a(6) = (6 / LeastPrimeFactor(6)) * LeastPrimeFactor(7) = 21.
		

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A126287, A126288, A126289, A020639.

Programs

Mathematica

a[1] := 2; a[n_] := n*FactorInteger[n + 1][[1, 1]]/FactorInteger[n][[1, 1]]; Table[a[n], {n, 69}] (* Ivan Neretin, May 20 2015 *)

PARI

a(n) = if (n==1, 2, n*factor(n+1)[1, 1]/factor(n)[1, 1]); \\ Michel Marcus, Aug 14 2013

Formula

a(n) = A032742(n)*A020639(n+1), for n>1. - Ivan Neretin, May 20 2015