Let $V$ be a finite dimensional vector space over an infinite field $k$. The ring of polynomial functions on $V$ is the subalgebra of the $k$-algebra of all functions $V\to k$ generated by the dual space $V^*$, and is denoted by $k[V]$.
Let $(e_1,\dots,e_n)$ be an ordered basis of $V$ and let $(f_1,\dots,f_n)$ be its dual basis, then an element of $k[V]$ is a polynomial in $f_1,\dots,f_n$. We can then define a (formal) derivative as follows: First, fix $i\in\{1,\dots,n\}$ and define $$ \partial_{e_i}(f_1^{r_1}\cdots f_{i-1}^{r_{i-1}}f_i^{r_i}f_{i+1}^{r_{i+1}}\cdots f_n^{r_n}) = r_i f_1^{r_1}\cdots f_{i-1}^{r_{i-1}}f_i^{r_i-1}f_{i+1}^{r_{i+1}}\cdots f_n^{r_n}, $$ for all $r_1,\dots,r_n\in \mathbb{Z}_{\geq 0}$. Extending by linearity we obtain a well defined derivation $\partial_{e_i}:k[V]\to k[V]$. Then for $v\in V$, write $$ v = \sum_{i=1}^n a_i e_i, \qquad a_1,\dots,a_n\in k $$ and define $$ \partial_v(f) = \sum_{i=1}^n a_i \partial_{e_i}(f), \qquad \forall f\in k[V]. $$
When we take $V=k^n$ and $(e_1,\dots,e_n)$ as the canonical ordered basis, the $i$-th vector in the dual basis is the coordinate function $x_i:k^n\to k$ given by $x_i(a_1,\dots,a_n) = a_i$, and $k[V]$ is precisely the polynomial ring $k[x_1,\dots,x_n]$ and the derivation $\partial_v$ coincides with the known formal directional derivative on that polynomial ring.
The main issue with this definition is that it depends on the chosen basis $(e_1,\dots,e_n)$. I would like to know if there is a basis-free definition of the derivative $\partial_v$ for a ring of polynomial functions $k[V]$ on a finite dimensional vector space $V$ over an infinite field $k$.
from Hot Weekly Questions - Mathematics Stack Exchange
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