A023585 Least prime divisor of 2*prime(n)-1.

3, 5, 3, 13, 3, 5, 3, 37, 3, 3, 61, 73, 3, 5, 3, 3, 3, 11, 7, 3, 5, 157, 3, 3, 193, 3, 5, 3, 7, 3, 11, 3, 3, 277, 3, 7, 313, 5, 3, 3, 3, 19, 3, 5, 3, 397, 421, 5, 3, 457, 3, 3, 13, 3, 3, 3, 3, 541, 7, 3, 5, 3, 613, 3, 5, 3, 661, 673, 3, 17, 3, 3, 733, 5, 757, 3, 3, 13, 3, 19, 3

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A020639, A076274.

Programs

Mathematica

FactorInteger[#][[1,1]]&/@(2Prime[Range[90]]-1) (* Harvey P. Dale, Oct 08 2017 *)

PARI

a(n) = factor(2*prime(n)-1)[1, 1]; \\ Michel Marcus, Oct 01 2013

Extensions

Definition edited by Michel Marcus, Oct 01 2013