A079747 Numbers k such that gpf(k-1) < gpf(k) < gpf(k+1), where gpf(k) is the greatest prime factor of k (A006530).

2, 9, 10, 21, 22, 25, 28, 33, 46, 57, 58, 78, 82, 85, 91, 92, 93, 106, 115, 121, 126, 133, 136, 141, 145, 148, 166, 169, 170, 171, 172, 176, 177, 178, 190, 201, 205, 213, 217, 221, 222, 226, 232, 235, 236, 248, 253, 261, 262, 276, 289, 290, 301, 316, 325, 346

Offset: 1

Author

Reinhard Zumkeller, Jan 10 2003

Keywords

• nonn

Comments

Numbers k such that A079748(k-1) > 1.

Examples

			k=25: 25-1 = 24 = 3*2^3, 25 = 5^2 and 25+1 = 26 = 13*2, therefore 25 is a term (3 < 5 < 13).
		

Links

• T. D. Noe, Table of n, a(n) for n=1..1000

Crossrefs

Cf. A071869, A071870, A082417-A082422.

Programs

Mathematica

gpf[n_] := FactorInteger[n][[-1, 1]]; ind = Position[Differences[Array[gpf, 350]], ?(# > 0 &)] // Flatten; ind[[Position[Differences[ind], 1] // Flatten]] + 1 (* _Amiram Eldar, Jun 05 2022 *)

Formula

a(n) = A071869(n-1) + 1. - T. D. Noe, Nov 26 2007