A073256 - OEIS

%I #18 Jul 05 2024 19:07:44

%S 2,2,4,6,4,4,6,8,8,6,8,12,10,8,20,12,18,12,8,16,20,16,24,12,18,24,16,

%T 12,20,24,22,16,42,20,32,24,18,40,24,36,28,16,30,36,32,48,20,32,44,24,

%U 24,36,40,36,60,24,32,54,40,24,64,42,56,40,24,72,44,60,46,72,32,42,60

%N a(n) = phi(n-th composite number).

%H Vincenzo Librandi, <a href="/A073256/b073256.txt">Table of n, a(n) for n = 1..1200</a>

%F a(n) = A000010(A002808(n)).

%e 100th composite is 133; phi(133) = 108 = a(100).

%t c[x_] := FixedPoint[x+PrimePi[ # ]+1&, x] Table[EulerPhi[c[w], {w, 1, 128}]

%t With[{nn=100},EulerPhi[#]&/@Complement[Range[2,nn], Prime[Range[ PrimePi[ nn]]]]] (* _Harvey P. Dale_, Apr 28 2014 *)

%t EulerPhi[Select[Range[100],CompositeQ]] (* _Harvey P. Dale_, Jul 05 2024 *)

%Y Cf. A000010, A002808, A035004, A073255.

%K nonn

%O 1,1

%A _Labos Elemer_, Jul 22 2002