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A056903
Numbers n such that the numerator of the rational number 1 + 1/2 + 1/3 + ... + 1/n is a prime number.
13
2, 3, 5, 8, 9, 21, 26, 41, 56, 62, 69, 79, 89, 91, 122, 127, 143, 167, 201, 230, 247, 252, 290, 349, 376, 459, 489, 492, 516, 662, 687, 714, 771, 932, 944, 1061, 1281, 1352, 1489, 1730, 1969, 2012, 2116, 2457, 2663, 2955, 3083, 3130, 3204, 3359, 3494, 3572
OFFSET
1,1
COMMENTS
Related to partial sums of the harmonic series and to Wolstenholme's Theorem.
Some of the larger entries may only correspond to probable primes.
LINKS
J. Sondow and E. W. Weisstein, MathWorld: Harmonic Number
Eric Weisstein's World of Mathematics, Integer Sequence Primes
EXAMPLE
5 is in this sequence because 1+1/2+1/3+1/4+1/5 = 137/60 and 137 is prime.
MATHEMATICA
Select[Range[1000], PrimeQ[Numerator[HarmonicNumber[ # ]]] &]
PROG
(Perl) use ntheory ":all"; for (1..1000) { say if is_prime((harmfrac($_))[0]); } # Dana Jacobsen, Feb 05 2016
(PARI) isok(n) = isprime(numerator(sum(k=1, n, 1/k))); \\ Michel Marcus, Feb 05 2016
CROSSREFS
Cf. A001008 (numerator of the harmonic number H(n)), A067657 (primes that are the numerator of a harmonic number).
Sequence in context: A120057 A099422 A294913 * A229139 A293277 A331864
KEYWORD
nonn
AUTHOR
EXTENSIONS
Terms from 201 to 492 computed by Jud McCranie.
More terms from Kamil Duszenko (kdusz(AT)wp.pl), Jun 22 2003
29 more terms from T. D. Noe, Sep 15 2004
Further terms found by Eric W. Weisstein, Mar 07 2005, Mar 29 2005, Nov 28 2005, Sep 23 2006
STATUS
approved