OFFSET
1,5
FORMULA
a(n) = (n*(n-1)/2)*Sum_{d|n} mu(d)*tau(d)/d, n > 1.
PROG
(PARI) a(n) = if (n==1, 1, n*(n-1)/2*sumdiv(n, d, moebius(d)*numdiv(d)/d)) \\ Michel Marcus, Jun 17 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Apr 29 2002
STATUS
approved