%I #12 Sep 08 2022 08:46:15

%S 0,2,2,8,8,12,18,32,24,40,50,48,72,84,56,128,128,108,162,160,120,220,

%T 242,192,240,312,234,336,392,240,450,512,320,544,408,432,648,684,456,

%U 640,800,504,882,880,528,1012,1058,768,1008,1000,800,1248,1352,972,1080,1344

%N Number of edges in the unitary addition Cayley graph Gn.

%C For n>1, the unitary addition Cayley graph Gn is the graph whose vertices are Z/nZ and where 2 vertices x and y are adjacent if x+y is a unit in Z/nZ.

%F a(n) = n*phi(n) if n is even, (n-1)*phi(n)/2 if n is odd.

%t Table[If[EvenQ@ n, n EulerPhi[n], (n - 1) EulerPhi[n]/2], {n, 56}] (* _Michael De Vlieger_, Feb 02 2016 *)

%o (PARI) a(n) = if (n % 2, eulerphi(n)*(n-1)/2, eulerphi(n)*n);

%o (Magma) [IsOdd(n mod 2) select (n-1)*(EulerPhi(n))/2 else n*(EulerPhi(n)): n in [1..52]]; // _Vincenzo Librandi_, Feb 03 2016

%Y Cf. A268341.

%K nonn

%O 1,2

%A _Michel Marcus_, Feb 02 2016