A124879 Numbers k such that A027612(k) is prime.

2, 3, 6, 9, 18, 25, 29, 30, 39, 84, 91, 125, 130, 184, 195, 199, 203, 241, 245, 273, 281, 378, 552, 571, 653, 776, 901, 1099, 1215, 1224, 1235, 1315, 1412, 1657, 1942, 2076, 2085, 2743, 2745, 2855, 2859, 3517, 3717, 4183, 4188, 4362, 4547, 4728, 4783

Offset: 1

Author

Alexander Adamchuk, Nov 11 2006

Keywords

• nonn

Examples

			A027612 begins {1, 5, 13, 77, 87, 223, 481, 4609, 4861, ...}.
Thus a(1) = 2, a(2) = 3, a(3) = 6, a(4) = 9.
		

Links

• Eric Weisstein's World of Mathematics, Harmonic Number.

Crossrefs

A027612(n) are the numerators of second order harmonic numbers H(n, (2)).

Corresponding primes in A027612 are listed in A124878.

Cf. A001008, A002805, A067657, A056903, A027612, A124878, A124837, A124880, A124881.

Programs

Mathematica

s=1;Do[s=s+1/(n+1);f=Numerator[(n+1)*(s-1)]; If[PrimeQ[f],Print[{n,f}]],{n,1,1942}]

PARI

isok(n) = isprime(numerator(sum(k=1, n, k/(n-k+1)))); \\ Michel Marcus, Jul 14 2018

Extensions

More terms from Stefan Steinerberger, May 29 2007

Crossrefs edited by Michel Marcus, Jul 14 2018