A128465 Numbers k such that k divides the numerator of alternating Harmonic number H'((k+1)/2) = A058313((k+1)/2).

1, 5, 7, 71, 379, 2659

Offset

1,2

Comments

For k > 1 all 5 listed terms are primes.

The only known numbers k such that k divides the numerator of alternating Harmonic number H'((k-1)/2) = A058313((k-1)/2) are the Wieferich primes (A001220): 1093 and 3511.

An odd prime p = prime(n) belongs to this sequence iff Fermat quotient A007663(n) == A130912(n) == 2*(-1)^((p+1)/2) (mod p). - Max Alekseyev, Nov 30 2022

Links

Table of n, a(n) for n=1..6.

Eric Weisstein's World of Mathematics, Harmonic Number

Mathematica

f=0; Do[ f = f + (-1)^(n+1)*1/n; g = Numerator[f]; If[ IntegerQ[ g/(2n-1) ], Print[2n-1]], {n, 1, 3000} ]

Crossrefs

Cf. A001008, A007663, A001220, A058313, A125854, A121999, A130912.

Sequence in context: A308397 A308848 A109715 * A098967 A107140 A141746

Adjacent sequences: A128462 A128463 A128464 * A128466 A128467 A128468

Keyword

hard,more,nonn

Author

Alexander Adamchuk, Mar 10 2007

Extensions

Edited by Max Alekseyev, Nov 30 2022

Status

approved