On the $Σ^0_1$-Completeness of Cubic Diophantine Systems and their Consequences for Equations
Milan Rosko
Abstract
We prove that $\mathrm{H}\mathbb{N}(3,n)=\mathrm{U}$ for some fixed $n$. The core result is a uniform primitive recursive compiler from sentence codes to finite cubic Diophantine systems over $\mathbb{N}$, such that arithmetic theoremhood is equivalent to cubic solvability on the compiler image. The translation is explicit: finite proof traces are arithmetized, local syntactic and inference checks are compiled to quadratic constraints, and one guard gadget $u = 1+v^2, u\cdot E = 0$ provides the unique degree escalation from $2$ to $3$ while preserving solvability. The construction is mechanized in $\textsf{Rocq}$ and certified in the Calculus of Inductive Constructions, including uniform computability, checker--constraint equivalence, and a global degree-$3$ bound for emitted systems. From this certified core, undecidability is obtained by a classical fixed-point Diagonal Argument: any total correct decider for cubic satisfiability computes a mirror cubic instance on which it fails. Consequently cubic satisfiability is $Σ^0_1$-complete, and in particular there is no total correct procedure deciding all degree-$3$ Diophantine instances.
