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

1,1

Links

Table of n, a(n) for n=1..49.

Eric Weisstein's World of Mathematics, Harmonic Number.

Example

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

Mathematica

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

Prog

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

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.

Sequence in context: A351255 A018402 A018441 * A261661 A062865 A351018

Adjacent sequences: A124876 A124877 A124878 * A124880 A124881 A124882

Keyword

nonn

Author

Alexander Adamchuk, Nov 11 2006

Extensions

More terms from Stefan Steinerberger, May 29 2007

Crossrefs edited by Michel Marcus, Jul 14 2018

Status

approved