Wikipedia:Reference desk/Archives/Mathematics/2016 September 24

= September 24 =

Injective and surjective polynomials
Let K be an algebraically closed field. Does a non-constant polynomial with coefficients in K necessarily define a surjective function from K to itself? Does a polynomial with coefficients in K defining an injective function from K to itself necessarily have degree 1? GeoffreyT2000 ( talk,  contribs ) 22:58, 24 September 2016 (UTC)


 * Yes to both questions: (1) if f(x) is a polynomial and a is in K, then f(x)-a splits. (2)  A degree n>1 polynomial that is one-to-one would have the property that f(x)-a is a perfect nth power for all a in K.  In particular, f(x)=x^n, and x^n-1=(x-1)^n.  This implies that n is a power of the characteristic of K, so f (x) is either the identity or is the Frobenius automorphism of a finite field.  No finite field is algebraically closed, a contradiction.  Sławomir Biały  (talk) 23:44, 24 September 2016 (UTC)
 * f(x) can also be the Frobenius automorphism of an infinite field of positive characteristic. And you can multiply it by a nonzero constant, and add a constant to it --77.125.79.22 (talk) 21:22, 30 September 2016 (UTC)