OFFSET
1,5
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..10000
EXAMPLE
n=15: sequence of D1 = {floor(15/j)} = {15,7,5,3,3,2,2,1,1,1,1,1,1,1,1}, Union(D1) = {15,7,5,3,2,1} = divisors(15) and {7,2}, a(15) = (15+7+5+3+2+1) - sigma(15) = 7 + 2 = 9.
MATHEMATICA
Table[Apply[Plus, Union[Table[Floor[w/j], {j, 1, w}]]] -DivisorSigma[1, w], {w, 1, 128}]
PROG
(PARI) a(n)=my(m=(sqrtint(4*n+1)-1)\2); m*(m+1)/2+sum(k=1, n\(m+1), n\k)-sigma(n) \\ Charles R Greathouse IV, Feb 14 2013
CROSSREFS
KEYWORD
nonn,look
AUTHOR
Labos Elemer, Nov 27 2002
STATUS
approved