A199423 Greatest prime factor of n and 2*n+1.

3, 5, 7, 3, 11, 13, 7, 17, 19, 7, 23, 5, 13, 29, 31, 11, 17, 37, 19, 41, 43, 11, 47, 7, 17, 53, 11, 19, 59, 61, 31, 13, 67, 23, 71, 73, 37, 19, 79, 5, 83, 17, 43, 89, 13, 31, 47, 97, 11, 101, 103, 13, 107, 109, 37, 113, 23, 29, 59, 11, 61, 31, 127, 43, 131, 19

Offset: 1

Author

Franklin T. Adams-Watters, Nov 06 2011

Keywords

• nonn
• easy

Links

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

Crossrefs

Cf. A006530, A014105, A074399, A093074, A076605.

Programs

Mathematica

Table[Max[Flatten[FactorInteger[{n,2 n+1}],1][[All,1]]],{n,70}] (* Harvey P. Dale, Mar 25 2020 *)

PARI

gpf(n)=local(ps);if(n<2,n,ps=factor(n)[,1]~;ps[#ps])
vector(80,n,gpf(n*(2*n+1)))

Formula

a(n) = A006530(A014105(n)).