%I
%S 1,2,3,4,5,5,6,7,8,9,10,10,11,12,13,14,15,15,16,16,17,18,19,19,20,21,
%T 22,22,23,23,24,25,26,27,28,28,29,30,31,31,32,32,33,34,35,36,37,37,38,
%U 39,40,41,42,42,43,43,44,45,46,46,47,48,49,50,51,51,52,53
%N Number of deficient numbers <= n.
%H Amiram Eldar, <a href="/A256562/b256562.txt">Table of n, a(n) for n = 1..10000</a>
%H Marc Deléglise, <a href="http://projecteuclid.org/euclid.em/1048515661">Bounds for the density of abundant integers</a>, Experiment. Math. Volume 7, Issue 2 (1998), 137143.
%H Charles R. Wall, Phillip L. Crews and Donald B. Johnson, <a href="http://dx.doi.org/10.1090/S00255718197203277007">Density bounds for the sum of divisors function</a>, Math. Comp. 26 (1972), 773777.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/AbundantNumber.html">Abundant Number</a>
%e For k=1 to 5, all numbers are deficients so a(k) = k in this range.
%e a(6) = 5 since 6 is the first number that is not deficient.
%t a[n_]:=Length[Select[Range[n],DivisorSigma[1,#]/#<2&]];a/@Range[68] (* _Ivan N. Ianakiev_, Apr 03 2015 *)
%o (PARI) a(n) = sum(k=1, n, sigma(k)/k < 2);
%o (MAGMA) [#[k:k in [1..n] DivisorSigma(1,k) lt 2*k]:n in [1..70]]; // _Marius A. Burtea_, Nov 06 2019
%Y Cf. A000396 (perfect), A005100 (deficient), A005101 (abundant).
%Y Cf. A091194 (number of abundant numbers <= n).
%Y Cf. A256440.
%K nonn
%O 1,2
%A _Michel Marcus_, Apr 02 2015
