 Hasse Derivative
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Hasse Derivative

In mathematics, the Hasse derivative is a generalisation of the derivative which allows the formulation of Taylor's theorem in coordinate rings of algebraic varieties.

## Definition

Let k[X] be a polynomial ring over a field k. The r-th Hasse derivative of Xn is

$D^{(r)}X^{n}={\binom {n}{r}}X^{n-r},$ if n >= r and zero otherwise. In characteristic zero we have

$D^{(r)}={\frac {1}{r!}}\left({\frac {\mathrm {d} }{\mathrm {d} X}}\right)^{r}\ .$ ## Properties

The Hasse derivative is a generalized derivation on k[X] and extends to a generalized derivation on the function field k(X), satisfying an analogue of the product rule

$D^{(r)}(fg)=\sum _{i=0}^{r}D^{(i)}(f)D^{(r-i)}(g)$ and an analogue of the chain rule. Note that the $D^{(r)}$ are not themselves derivations in general, but are closely related.

A form of Taylor's theorem holds for a function f defined in terms of a local parameter t on an algebraic variety:

$f=\sum _{r}D^{(r)}(f)\cdot t^{r}\ .$ 