%I #13 Aug 07 2021 16:37:35
%S 0,0,1,1,2,1,4,4,4,2,6,4,8,4,8,8,8,4,12,8,12,6,12,8,12,8,12,12,16,8,
%T 22,16,20,8,24,12,24,12,24,16,24,12,30,20,24,12,24,16,30,12,32,24,28,
%U 12,40,24,36,16,30,16,44,22,36,32,48,20,46,32,44,24
%N Euler totient function phi(n) - number of primitive roots modulo n.
%F a(n) = A000010(n) - A046144(n).
%p a:= proc(n) uses numtheory; `if`(n=1, 0, (p->
%p p-add(`if`(order(i, n)=p, 1, 0), i=0..n-1))(phi(n)))
%p end:
%p seq(a(n), n=1..70); # _Alois P. Heinz_, Jun 22 2021
%t a[n_] := (e = EulerPhi[n]) - If[n == 1 || IntegerQ @ PrimitiveRoot[n], EulerPhi[e], 0]; Array[a, 100] (* _Amiram Eldar_, Jun 23 2021 *)
%Y Cf. A000010, A046144.
%K nonn
%O 1,5
%A _Robert Hutchins_, Jun 22 2021
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