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Math Equations

Tuesday, May 9, 2017

Extremely rational cubic polynomials

By an extremely rational cubic polynomial, I mean a cubic polynomial p such that both p and p are products of rational linear factors. A parametrization will now be derived.

Seek to satisfy p=(a+x)(b+x)(c+x), p=3(d+x)(e+x) as polynomials.
0=p(e)=ab+ac+bc2(a+b+c)e+3e2(3eabc)2=(a+b+c)23(ab+ac+bc)=a2+b2+c2abacbcWe thus need to parametrize triples (a,b,c) such that the right-hand side is a square.

By the cubic number system, I mean triples (a,b,c) with the following operations.
(a,b,c)+(d,e,f)(a+d,b+e,c+f)(a,b,c)(d,e,f)(ad+bf+ce,ae+bd+cf,af+be+cd)Q(a,b,c)a2+b2+c2abacbcThis system is a close sibling of the usual complex number system. Its relevance to us is in the last definition. It is the expression we want to make into a square. Verify the identity Q(zw)=Q(z)Q(w) to realize that our task can be fulfilled by setting (a,b,c)(u,v,w)2. Normally, one would use fractions a,b,c,u,v,w in this number system, but we can just as well use complex parameters u,v,w.

Here is the parametrization.
p=(a+x)(b+x)(c+x),p=3(d+x)(e+x),p
Here is an example.
\begin{align*} (u,v,w)&=(1,2,7) \\ (a,b,c)&=(1^2+2\times 2\times 7,\ 7^2+2\times 1\times 2,\ 2^2+2\times 1\times 7)=(29,53,18) \\ p&=(29+x)(53+x)(18+x)=27666+3013x+100x^2+x^3 \\ Q(u,v,w)&=1^2+2^2+7^2-1\times 2-1\times 7-2\times 7=31 \\ Q(a,b,c)&=29\times 29+53\times 53+18\times 18-29\times 53-29\times 18-53\times 18=961 \\ d,e&=\frac13({10}^2\pm 31)=\frac{131}3,23\\ p'&=3(\frac{131}3+x)(23+x)=(131+3x)(69+x)=3013+200x+3x^2\\ f&=\frac{100}3,\quad p''=6(\frac{100}3+x)=200+6x \end{align*}
An example involving complex parameters u,v,w is best given visually and interactively.