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%I #19 Dec 21 2023 01:15:48
%S 1,2,4,4,6,8,8,8,12,12,12,16,14,16,22,16,18,24,20,24,30,24,24,32,30,
%T 28,36,32,30,44,32,32,46,36,46,48,38,40,54,48,42,60,44,48,66,48,48,64,
%U 56,60,70,56,54,72,70,64,78,60,60,88,62,64,90,64,82,92,68,72,94,92,72,96
%N Cototients of even numbers.
%C Not all even numbers arise as cototient values: see A005278.
%H Vincenzo Librandi, <a href="/A053196/b053196.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = 2n - phi(2n) = A051953(2n).
%F Sum_{k=1..n} a(k) ~ c * n^2 / 2, where c = 2 - 8/Pi^2 = 1.1894305... . - _Amiram Eldar_, Dec 21 2023
%t Table[(2 n - EulerPhi[2 n]), {n, 1, 80}] (* _Vincenzo Librandi_, Jul 27 2013 *)
%o (PARI) a(n) = 2*n - eulerphi(2*n) \\ _Michel Marcus_, Jul 26 2013
%o (Magma) [2*n-EulerPhi(2*n): n in [1..80]]; // _Vincenzo Librandi_, Jul 28 2013
%Y Cf. A000010, A051953, A005278.
%K nonn,easy
%O 1,2
%A _Labos Elemer_, Mar 02 2000
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