On the algebra of constants of polynomial derivations in two variables
Janusz Zieliński (2000)
Colloquium Mathematicae
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Let d be a k-derivation of k[x,y], where k is a field of characteristic zero. Denote by the unique extension of d to k(x,y). We prove that if ker d ≠ k, then ker = (ker d)0, where (ker d)0 is the field of fractions of ker d.