OFFSET
1,1
COMMENTS
Note a connection to the Wieferich primes A001220: a(2) = (A001220(1) - 1)/2, a(3) = A001220(1) - 1, a(4) = (A001220(2) - 1)/2, a(5) = A001220(2) - 1. [Comment regarding a(2) added by Kevin J. Gomez, Jul 11 2017]
a(9) > 840000. - Giovanni Resta, May 13 2016
MATHEMATICA
Select[Range[5000], GCD @@ Numerator@ HarmonicNumber@{#, Floor[#/2]} > 1 &] (* Giovanni Resta, May 13 2016 *)
PROG
(PARI) a001008(n)=numerator(sum(i=1, n, 1/i))
for(n=1, 1e6, if(gcd(a001008(n), a001008(n/2)) > 1, print1(n, ", "))) \\ Felix Fröhlich, Aug 08 2014
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Max Alekseyev and Tanya Khovanova, Mar 07 2007, corrected Mar 10 2007
EXTENSIONS
a(8) from Giovanni Resta, May 13 2016
STATUS
approved