OFFSET
1,4
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = card( k : sigma(k) < n ); a(n) is asymptotic to c*n with c=0.67...
MAPLE
N:= 100: # to get a(1)..a(N)
V:= Vector(N):
for n from 1 to N-2 do
s:= numtheory:-sigma(n)+1;
if s <= N then V[s]:= V[s]+1 fi;
od:
ListTools:-PartialSums(V); # Robert Israel, Jan 08 2018
MATHEMATICA
Table[Length[Select[Range[n], DivisorSigma[1, #] < n&]], {n, 1, 100}] (* Vaclav Kotesovec, Feb 16 2019 *)
PROG
(PARI) a(n)=sum(i=1, n, if(1+sign(sigma(i)-n), 0, 1))
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Sep 28 2002
STATUS
approved