+(*a new definition of totient, which uses sigma_p instead of sigma *)
+(*there's a little difference between this definition and the classic one:
+ the classic definition of totient is:
+
+ phi (n) is the number of naturals i (less or equal than n) so then gcd (i,n) = 1.
+ (so this definition considers the values i=1,2,...,n)
+
+ sigma_p doesn't work on the value n (but the first value it works on is (pred n))
+ but works also on 0. That's not a problem, in fact
+ - if n <> 1, gcd (n,0) <>1 and gcd (n,n) = n <> 1.
+ - if n = 1, then Phi(n) = 1, and (totient n), as defined below, returns 1.
+
+ *)