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In the algebraic mathematics, the Principal equation form also called Principal polynomial form is a special form of a polynomial equation. This form is of at least third degree and it does neither contain the term of the second highest degree nor the term of the third highest degree. In the computation of polynomial equations this form is used to simplify the given equations and to find out corresponding discriminants that characterize the equations.

Definition

The Principal form is a polynomial form of at least third degree in which the coefficients of the second highest and third highest degree are equal to zero. obeys this fundamental pattern:

${\displaystyle \sum _{m=0}^{n}c_{m}x^{m}=0\,\,}$

If a regular form of a polynomial equation is given, a corresponding principal form can be generated using Tschirnhaus transformations on the given equation, so that the degree of the Tschirnhaus key is smaller than the degree of the given equation. Especially the Tschirnhaus transformation with the holistic rational quadratic key and the transformation of a broken rational linear key can be used in order to create the principal form directly. In the following, the cubic, the quartic and the quintic polynomial equations of the principal form shall be analyzed accurately.

Cubic case

Tschirnhaus transformation with three clues

This is the given cubic equation:

${\displaystyle x^{3}-ax^{2}+bx-c=0}$

Following quadratic equation system shall be solved:

${\displaystyle \mathrm {First\,clue} }$	${\displaystyle au+3v+a^{2}-2b=0}$
${\displaystyle \mathrm {Second\,clue} }$	${\displaystyle bu^{2}-3v^{2}+abu-3cu-2ac+b^{2}=0}$
${\displaystyle \mathrm {Third\,clue} }$	${\displaystyle w=cu^{3}+acu^{2}+bcu+v^{3}+c^{2}}$

And so exactly this Tschirnhaus transformation appears:

${\displaystyle (x^{2}+ux+v)^{3}-w=0}$

Cubic calculation examples

Plastic constant:

${\displaystyle x^{3}-x-1=0}$

${\displaystyle {\bigl }^{3}-{\tfrac {23}{54}}(3{\sqrt {69}}-23)=0}$

Supergolden constant:

${\displaystyle x^{3}-x^{2}-1=0}$

${\displaystyle {\bigl }^{3}-{\tfrac {31}{18}}(29{\sqrt {93}}-279)=0}$

Tribonacci constant:

${\displaystyle x^{3}-x^{2}-x-1=0}$

${\displaystyle {\bigl }^{3}-{\tfrac {11}{72}}(19{\sqrt {33}}-99)=0}$

Quartic case

Tschirnhaus transformation with four clues

This is the given quartic equation:

${\displaystyle x^{4}-ax^{3}+bx^{2}-cx+d=0}$

Now this quadratic equation system shall be solved:

${\displaystyle \mathrm {First\,clue} }$	${\displaystyle at+4u+a^{2}-2b=0}$
${\displaystyle \mathrm {Second\,clue} }$	${\displaystyle bt^{2}-6u^{2}+abt-3ct-2ac+b^{2}+2d=0}$
${\displaystyle \mathrm {Third\,clue} }$	${\displaystyle v=ct^{3}+act^{2}-4dt^{2}-3adt+bct+4u^{3}-2bd+c^{2}}$
${\displaystyle \mathrm {Fourth\,clue} }$	${\displaystyle w=dt^{4}-u^{4}+uv+adt^{3}+bdt^{2}+cdt+d^{2}}$

And so accurately that Tschirnhaus transformation appears:

${\displaystyle (x^{2}+tx+u)^{4}-v(x^{2}+tx+u)+w=0}$

Quartic calculation examples

The Tschirnhaus transformation of the equation for the Tetranacci constant contains only rational coefficients:

${\displaystyle x^{4}-x^{3}-x^{2}-x-1=0}$

${\displaystyle y=x^{2}-3x}$

${\displaystyle y^{4}-11y-41=0}$

In this way following expression can be made about the Tetranacci constant:

${\displaystyle x^{2}-3x=({\tfrac {41}{3}})^{1/4}{\sqrt {\sinh {\bigl }}}-}$

${\displaystyle -({\tfrac {41}{3}})^{1/4}{\bigl \{}{\tfrac {11}{4}}({\tfrac {3}{41}})^{3/4}{\sqrt {\operatorname {csch} {\bigl }}}-\sinh {\bigl }{\bigr \}}^{1/2}}$

That calculation example however does contain the element of the square root in the Tschirnhaus transformation:

${\displaystyle x^{4}+x^{3}+x^{2}-x-1=0}$

${\displaystyle y=x^{2}+{\tfrac {1}{5}}(19+4{\sqrt {21}})x+{\tfrac {1}{5}}(6+{\sqrt {21}})}$

${\displaystyle y^{4}-{\tfrac {1}{125}}(38267+8272{\sqrt {21}})y-{\tfrac {1}{625}}(101277{\sqrt {21}}+463072)=0}$

Special form of the quartic

In the following we solve a special equation pattern that is easily solvable by using elliptic functions:

${\displaystyle x^{4}-6x^{2}-8{\sqrt {S^{2}+1}}\,x-3=0}$

${\displaystyle Q=q{\bigl \{}\tanh {\bigl }{\bigr \}}=q{\bigl }}$

${\displaystyle x={\frac {3\,\vartheta _{01}(Q^{3})^{2}}{\vartheta _{01}(Q)^{2}}}}$

These are important additional informations about the elliptic nome and the mentioned Jacobi theta function:

${\displaystyle q(\varepsilon )=\exp {\bigl }}$

${\displaystyle \vartheta _{01}(r)=\sum _{k=-\infty }^{\infty }(-1)^{k}r^{k^{2}}=\prod _{n=1}^{\infty }(1-r^{2n})(1-r^{2n-1})^{2}}$

Computation rule for the mentioned theta quotient:

${\displaystyle {\frac {3\,\vartheta _{01}\{q^{3}\}^{2}}{\vartheta _{01}\{q\}^{2}}}={\sqrt {2{\sqrt {\kappa ^{4}-\kappa ^{2}+1}}-\kappa ^{2}+2}}+{\sqrt {\kappa ^{2}+1}}}$

Accurately the Jacobi theta function is used for solving that equation.

Now we create a Tschirnhaus transformation on that:

${\displaystyle x^{4}-6x^{2}-8{\sqrt {S^{2}+1}}\,x-3=0}$

${\displaystyle y=x^{2}-2({\sqrt {S^{2}+1}}-S)x-3}$

${\displaystyle y^{4}+64\,S^{2}(4S^{2}+1-4S{\sqrt {S^{2}+1}})y-384\,S^{3}({\sqrt {S^{2}+1}}-S)=0}$

Elliptic solving of principal quartics

Now this solving pattern shall be used for solving some principal quartic equations:

First calculation example:

${\displaystyle x^{4}+x-1=0}$

${\displaystyle Q=q{\bigl \{}\tanh {\bigl }{\bigr \}}}$

${\displaystyle x={\frac {4}{{\sqrt {2{\sqrt {849}}+18}}-6}}{\biggl }}$

Second calculation example:

${\displaystyle x^{4}+2x-1=0}$

${\displaystyle Q=q{\bigl \{}\tanh {\bigl }{\bigr \}}}$

${\displaystyle x={\frac {2}{{\sqrt {2{\sqrt {129}}+18}}-6}}{\biggl }}$

Third calculation example:

${\displaystyle x^{4}+5x-3=0}$

${\displaystyle Q=q{\bigl \{}\tanh {\bigl }{\bigr \}}}$

${\displaystyle x={\frac {4}{{\sqrt {2{\sqrt {881}}+50}}-10}}{\biggl }}$

Clue for the computation of the S value for a given principal quartic:

${\displaystyle x^{4}+\psi x-\omega =0}$

If this equation pattern is given, the modulus tangent duplication value S can be determined in this way:

${\displaystyle \psi ^{4}{\bigl }^{3}=\omega ^{3}{\bigl }^{4}}$

The tangential halving of S leads to the elliptic modulus from which the Elliptic nome has to be calculated. The solution of the now mentioned formula always is in pure biquadratic radical relation to psi and omega and therefore it is a useful tool to solve principal quartic equations.

Quintic case

Synthesis advice for the quadratic Tschirnhaus key

This is the given quintic equation:

${\displaystyle x^{5}-ax^{4}+bx^{3}-cx^{2}+dx-e=0}$

That quadratic equation system leads to the coefficients of the quadratic Tschirnhaus key:

${\displaystyle \mathrm {First\,clue} }$	${\displaystyle as+5t+a^{2}-2b=0}$
${\displaystyle \mathrm {Second\,clue} }$	${\displaystyle bs^{2}-10t^{2}+abs-3cs-2ac+b^{2}+2d=0}$

By polynomial division that Tschirnhaus transformation can be made:

${\displaystyle (x^{2}+sx+t)^{5}-u(x^{2}+sx+t)^{2}+v(x^{2}+sx+t)-w=0}$

Calculation examples

This is the first example:

${\displaystyle x^{5}-x^{4}-x^{2}-1=0}$

${\displaystyle y=x^{2}-{\tfrac {1}{4}}(19-{\sqrt {265}})x-{\tfrac {1}{20}}({\sqrt {265}}-15)}$

${\displaystyle y^{5}+{\tfrac {1}{80}}(24455-1501{\sqrt {265}})y^{2}-{\tfrac {1}{160}}(5789{\sqrt {265}}-93879)y-{\tfrac {1}{4000}}(5393003{\sqrt {265}}-87785025)=0}$

And this is the second example:

${\displaystyle x^{5}+x^{4}+x^{3}+x^{2}-1=0}$

${\displaystyle y=x^{2}+{\tfrac {1}{3}}({\sqrt {30}}-3)x+{\tfrac {1}{15}}{\sqrt {30}}}$

${\displaystyle y^{5}-{\tfrac {1}{45}}(465-61{\sqrt {30}})y^{2}+{\tfrac {2}{45}}(1616-289{\sqrt {30}})y-{\tfrac {1}{1125}}(33758{\sqrt {30}}-183825)=0}$

Solving the principal quintic via Adamchik transformation

The mathematicians Victor Adamchik and David Jeffrey found out how to solve every principal quintic equation. In their essay Polynomial Transformations of Tschirnhaus, Bring and Jerrard they wrote this way down. These two mathematicians solved this principal form by transforming it into the Bring Jerrard form. Their method contains the construction of a quartic Tschirnhaus transformation key. For the construction of that key they executed a disjunction of the linear term coefficient of the key in order to get a system that solves all other terms in a quadratic radical way and to only solve a further cubic equation to get the coefficient of the linear term of the Tschirnhaus key.

In their essay they constructed the quartic Tschirnhaus key in this way:

${\displaystyle y^{5}-uy^{2}+vy-w=0}$

In order to do the transformation Adamchik and Jeffrey constructed equation system that generates the coefficients of the cubic, quadratic and absolute term of the Tschirnhaus key:

${\displaystyle 4v\alpha -3u\beta =5w}$

${\displaystyle 3u\alpha +5\delta =4v}$

${\displaystyle 15w\alpha \beta -3u\alpha \delta -6v\beta ^{2}-v\delta =3uw-2v^{2}}$

And for receiving the coefficient of the linear term this cubic equation shall be solved successively:

${\displaystyle u\gamma ^{3}+(5w\alpha -4v\beta +3u^{2})\gamma ^{2}+(uv\alpha ^{2}-8dv\alpha +5w\beta ^{2}+6du\beta -8uv\beta +3u^{3}+9vw)\gamma +}$

${\displaystyle +u^{3}\alpha ^{3}+vw\alpha ^{3}+u^{2}\beta ^{3}-10uw\alpha ^{2}\beta +4v^{2}\alpha ^{2}\beta +9u^{2}\alpha ^{2}\delta -uv\alpha \beta ^{2}+15u\alpha \delta ^{2}+10\delta ^{3}-}$

${\displaystyle -u^{2}v\alpha ^{2}-3u^{3}\alpha \beta +6vw\alpha \beta -12uv\alpha \delta +5uw\beta ^{2}-4v^{2}\beta ^{2}+uv^{2}\alpha -u^{2}w\alpha +2u^{2}v\beta +u^{4}-4v^{3}+8uvw=0}$

Examples of solving the principal form

Along with the Abel Ruffini theorem the following equation is an example that can not be solved in an elementary way, but can be reduced to the Bring Jerrard form by only using cubic radical elements. This shall be demonstrated here:

${\displaystyle y^{5}-5y^{2}+5y-5=0}$

For this given equation the quartic Tschirnhaus key now shall be synthesized. For that example the values u = v = w = 5 are the combination. Therefore this is the mentioned equation system for that example:

${\displaystyle 20\alpha -15\beta =25}$

${\displaystyle 15\alpha +5\delta =20}$

${\displaystyle 75\alpha \beta -15\alpha \delta -30\beta ^{2}-5\delta =25}$

So these are the coefficients of the cubic, quadratic and absolute term of the Tschirnhaus transformation key:

${\displaystyle \alpha =-1,\,\beta =-3,\,\delta =7}$

That cubic equation leads to the coefficient of the linear term of the key:

${\displaystyle 5\gamma ^{3}+110\gamma ^{2}+1100\gamma +3080=0}$

By doing a polynomial division on the fifth power of the quartic Tschirnhaus transformation key and analyzing the remainder rest the coefficients of the mold can be determined. This is the result:

${\displaystyle y^{5}-5y^{2}+5y-5=0}$

${\displaystyle z=y^{4}-y^{3}-3y^{2}+{\bigl \{}{\frac {8}{3}}{\sqrt {11}}\sinh {\bigl }-{\frac {22}{3}}{\bigr \}}y+7}$

${\displaystyle z^{5}+{\frac {14080}{3}}{\bigl \{}11-2{\sqrt {22}}\cosh {\bigl }{\bigr \}}z+{\frac {11264}{3}}{\bigl \{}11-110{\sqrt {2}}\sinh {\bigl }{\bigr \}}=0}$

These are the approximations of the solution:

${\displaystyle z\approx -4.87187987090341241739191116705958390845844658170795795268900739402026742}$

${\displaystyle y\approx 1.56670895425072582758152133323240667646412076995364965189840377191745}$

Literature

• Victor Adamchik, David Jeffrey: Archived (Date missing) at sigsam.org (Error: unknown archive URL), ACM Sigsam Bulletin, Band 37, 2003

• F. Brioschi: Sulla risoluzione delle equazioni del quinto grado: Hermite — Sur la résolution de l'Équation du cinquiéme degré Comptes rendus —. N. 11. Mars. 1858. 1. Dezember 1858, doi:10.1007/bf03197334

• Bruce and King: Beyond the Quartic Equation, publisher=Birkhäuser, ISBN = 3-7643-3776-1

References

Algebra Polynomials Nonlinear systems

1. https://mathworld.wolfram.com/PrincipalQuinticForm.html
2. https://math.stackexchange.com/questions/4725226/the-solution-to-the-principal-quintic-via-the-brioschi-and-rogers-ramanujan-cfra
3. Jerrard, George Birch (1859). An essay on the resolution of equations. London, UK: Taylor & Francis.
4. Adamchik, Victor (2003). "Polynomial Transformations of Tschirnhaus, Bring, and Jerrard" (PDF). ACM SIGSAM Bulletin. 37 (3): 91. CiteSeerX 10.1.1.10.9463. doi:10.1145/990353.990371. S2CID 53229404. Archived from the original (PDF) on 2009-02-26.
5. https://www.uwo.ca/apmaths/faculty/jeffrey/pdfs/Adamchik.pdf
6. https://math.stackexchange.com/questions/4988153/a-new-way-to-solve-the-bring-quintic
7. https://www.oocities.org/titus_piezas/Tschirnhausen.html
8. https://archive.org/details/cu31924059413439/page/n181/mode/2up

• Algebra
• Equations
• Polynomials
• Polynomial functions
• Galois theory
• Nonlinear systems