A199423 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

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

FORMULA

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

MATHEMATICA

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

PROG

(PARI) gpf(n)=local(ps); if(n<2, n, ps=factor(n)[, 1]~; ps[#ps])

vector(80, n, gpf(n*(2*n+1)))