A027761 Numerator of sum_{p prime, p-1 divides 2*n} 1/p.

5, 31, 41, 31, 61, 3421, 5, 557, 821, 371, 121, 3421, 5, 929, 15745, 557, 5, 2557843, 5, 15541, 1805, 743, 241, 60887, 61, 1673, 821, 929, 301, 79085411, 5, 557, 66961, 31, 4397, 188641729, 5, 31, 3281, 277727, 421, 4462547, 5, 66817, 313477, 1487, 5, 5952449

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• frac

Links

• Seiichi Manyama, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A027762.

Programs

Mathematica

a[n_] := Sum[ Boole[ PrimeQ[d+1] ]/(d+1), {d, Divisors[2n]}] // Numerator; Table[a[n], {n, 1, 48}] (* Jean-François Alcover, Aug 10 2012 *)

PARI

a(n)=
{
    my(bd=0);
    forprime (p=2, 2*n+1, if( (2*n)%(p-1)==0, bd += 1/p;  ) );
    bd = numerator(bd);
    return(bd);
}
/* Joerg Arndt, May 06 2012 */