%I #23 Apr 20 2018 00:38:46
%S 2,5,5,9,9,13,13,18,18,20,20,26,26,26,26,32,32,36,36,41,41,43,43,53,
%T 53,53,53,55,55,57,57,64,64,64,64,72,72,72,72,81,81,85,85,88,88,90,90,
%U 101,101,101,101,103,103,105,105,108,108,110,110,119,119,119,119,127,127
%N a(n) = Card{ k, phi(k) <= n }.
%D G. Tenenbaum & Jie Wu, Exercices corrigés de théorie analytique et probabiliste des nombres, Collection SMF, Cours specialises, Numero 2, pp. 78-79.
%D S. R. Finch, Mathematical Constants, Cambridge, 2003, pp. 115-118.
%H T. D. Noe, <a href="/A070243/b070243.txt">Table of n, a(n) for n = 1..1000</a>
%H Matteo Caorsi, Sergio Cecotti, <a href="https://arxiv.org/abs/1801.04542">Geometric classification of 4d N=2 SCFTs</a>, arXiv:1801.04542 [hep-th], 2018.
%F Lim_{n ->infinity} a(n)/n = zeta(2)zeta(3)/zeta(6) = 1.943596436820759205057... = A082695.
%F a(n) = Sum_{k=1..n} A014197(k); a(n) = zeta(2)*zeta(3)/zeta(6)*n + O(n*exp(-c*sqrt(log(n))) for a suitable constant c > 0. - _Benoit Cloitre_, Apr 12 2003
%o (PARI) for(n=1,150,print1(sum(i=1,100*n,if(sign(eulerphi(i)-n)+1,0,1)+if((eulerphi(i)-n),0,1)),","))
%K easy,nonn
%O 1,1
%A _Benoit Cloitre_, May 11 2002