On the polynomial values represented by quadratic forms

Many Diophantine equations can be reduced to the question of whether, for a given non-degenerate quadratic form $F$ and a univariate polynomial $P$ with integer coefficients, $P(x)$ can be represented by $F$ for infinitely many values of $x$.

We develop a method for answering this question for certain cubic and quartic polynomials $P$, as well as for certain polynomials of the form $P(x)=R(Q(x))$, where $R(t)$ and $Q(x)$ are polynomials of degree $3$ and $2$, respectively.

Applying this method with $F(y,z)=y^2+z^2$, $R(t)=t^3-4$ and $Q(x)=x^2$, we conclude that $x^6-4$ is a sum of two squares infinitely often.

In turn, this implies that the equation $y^2+x^3y+z^2+1=0$ has infinitely many integer solutions.

Prior to this work, it was the shortest equation for which it was open whether its integer solution set is finite or infinite.

We conclude with a list of the new shortest equations whose finiteness problem remains open.

All main results of this paper has been formalized in Lean using Aristotle.