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+bc−2(a+b+c)e+3e2(3e−a−b−c)2=(a+b+c)2−3(ab+ac+bc)=a2+b2+c2−ab−ac−bcWe 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+c2−ab−ac−bcThis 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.