A076565 Greatest prime divisor of 2n+1 (sum of two successive integers).

3, 5, 7, 3, 11, 13, 5, 17, 19, 7, 23, 5, 3, 29, 31, 11, 7, 37, 13, 41, 43, 5, 47, 7, 17, 53, 11, 19, 59, 61, 7, 13, 67, 23, 71, 73, 5, 11, 79, 3, 83, 17, 29, 89, 13, 31, 19, 97, 11, 101, 103, 7, 107, 109, 37, 113, 23, 13, 17, 11, 41, 5, 127, 43, 131, 19, 5, 137, 139, 47, 13

Offset: 1

Author

Zak Seidov, Oct 19 2002

Keywords

• easy
• nonn

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A006530.

Programs

Maple

seq(max(numtheory:-factorset(2*n+1)),n=1..200); # Robert Israel, Mar 17 2017

Mathematica

Table[FactorInteger[2n + 1][[-1,1]], {n,1, 100}] (* Indranil Ghosh, Mar 20 2017 *)

PARI

for(n=1, 100, print1(vecmax(factor(2*n + 1)[,1]),", ")) \\ Indranil Ghosh, Mar 20 2017

Python

from sympy.ntheory import primefactors
print([max(primefactors(2*n + 1)) for n in range(1,101)]) # Indranil Ghosh, Mar 20 2017

Formula

a(n) = A006530(2n+1). - Robert Israel, Mar 17 2017