Tschirnhaus transformations

A polynomial transformation which transforms a polynomial to another with certain zero-coefficients is called a Tschirnhaus Transformation. It is thus an invertible transformation of the form $x\mapsto g(x)/h(x)$ where $g,h$ are polynomials over the base field $K$ (or some subfield of the splitting field of the polynomial being transformed). If $\gcd (h(x),f(x))=1$ then the Tschirnhaus transformation becomes a polynomial transformation mod f.

Specifically, it concerns a substitution that reduces finding the roots of the polynomial

$$\text{p}=T^n+a_1{T}^{n-1}+\mathrm{\dots }+a_n=\prod _{i=1}^n(T-r_i)\in k[T]$$

to finding the roots of another q - with less parameters - and solving an auxiliary polynomial equation s, with

Historically, the transformation was applied to reduce the general quintic equation, to simpler resolvents. Examples due to Hermite and Klein are respectively: The principal resolvent

$$K(X):=X^5+a_0{X}^2+a_{1}X+a_3$$

and the Bring-Jerrard form

$$K(X):=X^5+a_{1}X+a_2$$

Tschirnhaus transformations are also used when computing Galois groups to remove repeated roots in resolvent polynomials. Almost any transformation will work but it is extremely hard to find an efficient algorithm that can be proved to work.

Title	Tschirnhaus transformations
Canonical name	TschirnhausTransformations
Date of creation	2013-03-22 13:50:12
Last modified on	2013-03-22 13:50:12
Owner	mathcam (2727)
Last modified by	mathcam (2727)
Numerical id	11
Author	mathcam (2727)
Entry type	Definition
Classification	msc 12E05
Synonym	Tschirnhausen Transformation
Related topic	QuadraticResolvent
Related topic	EulersDerivationOfTheQuarticFormula