OFFSET
1,2
COMMENTS
Not all even numbers arise as cototient values: see A005278.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = 2n - phi(2n) = A051953(2n).
Sum_{k=1..n} a(k) ~ c * n^2 / 2, where c = 2 - 8/Pi^2 = 1.1894305... . - Amiram Eldar, Dec 21 2023
MATHEMATICA
Table[(2 n - EulerPhi[2 n]), {n, 1, 80}] (* Vincenzo Librandi, Jul 27 2013 *)
PROG
(PARI) a(n) = 2*n - eulerphi(2*n) \\ Michel Marcus, Jul 26 2013
(Magma) [2*n-EulerPhi(2*n): n in [1..80]]; // Vincenzo Librandi, Jul 28 2013
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Labos Elemer, Mar 02 2000
STATUS
approved