Tschirnhaus transformation
In mathematics, a Tschirnhaus transformation, also known as Tschirnhausen transformation, is a type of mapping on polynomials developed by Ehrenfried Walther von Tschirnhaus in 1683.<ref name=":1">Template:Cite journal</ref>
Simply, it is a method for transforming a polynomial equation of degree <math>n\ge2</math> with some nonzero intermediate coefficients, <math>a_1, ..., a_{n-1}</math>, such that some or all of the transformed intermediate coefficients, <math>a'_1, ..., a'_{n-1}</math>, are exactly zero.
For example, finding a substitution<math display="block">y(x)=k_1x^2 + k_2x+k_3</math>for a cubic equation of degree <math>n=3</math>,<math display="block">f(x) = x^3+a_2x^2+a_1x+a_0</math>such that substituting <math>x=x(y)</math> yields a new equation<math display="block">f'(y)=y^3+a'_2y^2+a'_1y+a'_0</math>such that <math>a'_1=0</math>, <math>a'_2=0</math>, or both.
More generally, it may be defined conveniently by means of field theory, as the transformation on minimal polynomials implied by a different choice of primitive element. This is the most general transformation of an irreducible polynomial that takes a root to some rational function applied to that root.
Definition
For a generic <math>n^{th}</math> degree reducible monic polynomial equation <math>f(x)=0</math> of the form <math>f(x) = g(x) / h(x)</math>, where <math>g(x)</math> and <math>h(x)</math> are polynomials and <math>h(x)</math> does not vanish at <math>f(x) = 0</math>,<math display="block">f(x) = x^n+a_1x^{n-1}+a_2x^{n-2}+...+a_{n-1}x+a_n=0</math>the Tschirnhaus transformation is the function:<math display="block">y=k_1x^{n-1} + k_2x^{n-2}+...+k_{n-1}x+k_n</math>Such that the new equation in <math>y</math>, <math>f'(y)</math>, has certain special properties, most commonly such that some coefficients, <math>a'_1,...,a'_{n-1}</math>, are identically zero.<ref>Template:Cite journal</ref><ref name=":0">C. B. Boyer (1968) A History of Mathematics. Wiley, New York pp. 472-473. As reported by: Template:Cite web</ref>
Example: Tschirnhaus' method for cubic equations
In Tschirnhaus' 1683 paper,<ref name=":1" /> he found the roots of the polynomial <math display="block">f(x)=x^3-px^2+qx-r</math> using the change of variables <math display="block">y(x;a)=x-a</math> and its inverse <math display="block">x(y;a)=y+a.</math> Replacing <math>x</math> by <math>y+a</math> in <math>f</math>, and expanding the powers of <math>y+a</math> in the resulting formula, yields the transformed polynomial<math display="block"> \begin{align} f'(y;a)&=y^3+(3a-p)y^2+(3a^2-2pa+q) y+(a^3-pa^2+qa-r)\\ &=y^3+a'_1y^2+a'_2y+a'_3 \end{align}</math> with the coefficients <math display="block">\begin{align} a'_1&=3a-p \\ a'_2&=3a^2-2pa+q \\ a'_3&=a^3-pa^2+qa-r. \end{align}</math> The quadratic term in <math>f'</math> may be eliminated by setting <math>a'_1=0</math>, and solving for <math>a'_1=3a-p=0</math> determines the parameter <math>a</math> as <math>a=p/3</math>. Thus, the Tschirnhaus transformation <math display="block">y=x-\frac{p}{3},</math> may be substituted into <math>f'(y;a)</math> to yield a polynomial of the form <math display="block">f'(y)=y^3-q'y-r'.</math> The roots of the original polynomial <math>f</math> may be obtained from the roots of this transformed polynomial <math>f'</math> by the same transformation. Tschirnhaus went on to describe how a Tschirnhaus transformation of the form <math display="block">x^2=bx+y+a</math> may be used to eliminate two coefficients in a similar way.
Generalization
In detail, let <math>K</math> be a field, and <math>P(t)</math> a polynomial over <math>K</math>. If <math>P</math> is irreducible, then the quotient ring of the polynomial ring <math>K[t]</math> by the principal ideal generated by <math>P</math>,
- <math>K[t]/(P(t)) = L</math>,
is a field extension of <math>K</math>. We have
- <math>L = K(\alpha)</math>
where <math>\alpha</math> is <math>t</math> modulo <math>(P)</math>. That is, any element of <math>L</math> is a polynomial in <math>\alpha</math>, which is thus a primitive element of <math>L</math>. There will be other choices <math>\beta</math> of primitive element in <math>L</math>: for any such choice of <math>\beta</math> we will have by definition:
- <math>\beta = F(\alpha), \alpha = G(\beta)</math>,
with polynomials <math>F</math> and <math>G</math> over <math>K</math>. Now if <math>Q</math> is the minimal polynomial for <math>\beta</math> over <math>K</math>, we can call <math>Q</math> a Tschirnhaus transformation of <math>P</math>.
Therefore the set of all Tschirnhaus transformations of an irreducible polynomial is to be described as running over all ways of changing <math>P</math>, but leaving <math>L</math> the same. This concept is used in reducing quintics to Bring–Jerrard form, for example. There is a connection with Galois theory, when <math>L</math> is a Galois extension of <math>K</math>. The Galois group may then be considered as all the Tschirnhaus transformations of <math>P</math> to itself.
History
In 1683, Ehrenfried Walther von Tschirnhaus published a method for rewriting a polynomial of degree <math>n>2</math> such that the <math>x^{n-1}</math> and <math>x^{n-2}</math> terms have zero coefficients. In his paper, Tschirnhaus referenced a method by René Descartes to reduce a quadratic polynomial <math>(n=2)</math> such that the <math>x</math> term has zero coefficient.
In 1786, this work was expanded by Erland Samuel Bring who showed that any generic quintic polynomial could be similarly reduced.
In 1834, George Jerrard further expanded Tschirnhaus' work by showing a Tschirnhaus transformation may be used to eliminate the <math>x^{n-1}</math>, <math>x^{n-2}</math>, and <math>x^{n-3}</math> for a general polynomial of degree <math>n>3</math>.<ref name=":0" />
See also
- Completing the square
- Polynomial transformations
- Monic polynomial
- Reducible polynomial
- Quintic function
- Galois theory
- Abel–Ruffini theorem
- Principal equation form