Article
Keywords:
higher derivation; field extension; closed polynomial
Summary:
Let $k\subseteq k'$ be a field extension. We give relations between the kernels of higher derivations on $k[X]$ and $k'[X]$, where $k[X]:=k[x_1,\dots ,x_n]$ denotes the polynomial ring in $n$ variables over the field $k$. More precisely, let $D=\{D_n\}_{n=0}^\infty $ a higher $k$-derivation on $k[X]$ and $D'=\{D_n'\}_{n=0}^\infty $ a higher $k'$-derivation on $k'[X]$ such that $D'_m(x_i)=D_m(x_i)$ for all $m\geq 0$ and $i=1,2,\dots ,n$. Then (1) $k[X]^D=k$ if and only if $k'[X]^{D'}=k'$; (2) $k[X]^D$ is a finitely generated $k$-algebra if and only if $k'[X]^{D'}$ is a finitely generated $k'$-algebra. Furthermore, we also show that the kernel $k[X]^D$ of a higher derivation $D$ of $k[X]$ can be generated by a set of closed polynomials.
References:
                        
[4] Miyanishi, M.: 
Lectures on Curves on Rational and Unirational Surfaces. Tata Institute of Fundamental Research Lectures on Mathematics and Physics Berlin, Springer (1978). 
MR 0546289 | 
Zbl 0425.14008 
[5] Nowicki, A.: 
Polynomial Derivations and their Rings of Constants. N. Copernicus Univ. Press Toruń (1994). 
MR 2553232 | 
Zbl 1236.13023 
[6] Roman, S.: 
Advanced Linear Algebra. 3rd edition, Graduate Texts in Mathematics 135 New York, Springer (2008). 
MR 2344656 | 
Zbl 1132.15002