Function: eulerphi
Section: number_theoretical
C-Name: eulerphi
Prototype: G
Help: eulerphi(x): Euler's totient function of x.
Description:
 (gen):int        eulerphi($1)
Doc: Euler's $\phi$ (totient)\sidx{Euler totient function} function of the
 integer $|x|$, in other words $|(\Z/x\Z)^{*}|$.
 \bprog
 ? eulerphi(40)
 %1 = 16
 @eprog\noindent
 According to this definition we let $\phi(0) := 2$, since $\Z^{*} = \{-1,1\}$;
 this is consistent with \kbd{znstar(0)}: we have
 \kbd{znstar$(n)$.no = eulerphi(n)} for all $n\in\Z$.
