A062771 - OEIS

%I #30 Dec 30 2022 03:55:01

%S 1,6,2,8,4,12,6,16,6,24,10,16,12,36,8,32,16,36,18,32,12,60,22,32,20,

%T 72,18,48,28,48,30,64,20,96,24,48,36,108,24,64,40,72,42,80,24,132,46,

%U 64,42,120,32,96,52,108,40,96,36,168,58,64,60,180,36,128,48,120,66,128,44

%N Order of automorphism group of the group C_n X C_2 (where C_n is the cyclic group with n elements).

%H David A. Corneth, <a href="/A062771/b062771.txt">Table of n, a(n) for n = 1..10000</a>

%F For odd n: a(n) = phi(n) (sequence A000010).

%F Conjecture: a(n) = 6*phi(n) if n mod 4 = 2 and a(n) = 4*phi(n) if n mod 4 = 0. - _Vladeta Jovovic_, Jul 20 2001

%F Conjecture confirmed. - _Christian G. Bower_, May 20 2005

%F Multiplicative with a(2) = 6, a(2^e) = 2^(e+1), e>1, a(p^e) = (p-1)*p^(e-1), p>2. - _Christian G. Bower_, May 18 2005

%F Sum_{k=1..n} a(k) ~ c * n^2, where c = 7/Pi^2 = 0.709248... . - _Amiram Eldar_, Oct 30 2022

%F Dirichlet g.f.: (zeta(s-1)/zeta(s))*((2^s-4/2^s+4)/(2^s-1)). - _Amiram Eldar_, Dec 30 2022

%t a[n_] := Switch[Mod[n, 4], 0, 4, 2, 6, _, 1]*EulerPhi[n];

%t Array[a, 69] (* _Jean-François Alcover_, Apr 19 2018 *)

%o (PARI) a(n)=my(p=eulerphi(n)); if(n%2==1, p, if(n%4==2,6*p,4*p)); \\ _Joerg Arndt_, Sep 09 2020

%Y Cf. A000010.

%K nonn,easy,mult

%O 1,2

%A Ahmed Fares (ahmedfares(AT)my-deja.com), Jul 18 2001

%E More terms from _Christian G. Bower_, May 20 2005