Sometimes systems of Diophantine equations are considered.

Problems generally involve finding integer solutions to the equations, notably determining if any solution exists, and if infinitely solutions exist.

The general problem is known to be undecidable: Hilbert's tenth problem.

The Pythagorean triples, and its generalization Fermat's last theorem, are the quintessential examples.

Direct consequence of Euclid's formula.

A generalization of the Pythagorean triple infinity question.

Once you hear about the uncomputability of such problems, it makes you see that all Diophantine equation questions risk being undecidable, though in some simpler cases we manage to come up with answers. The feeling is similar to watching people trying to solve the Halting problem, e.g. in the effort to determine BB(5).

www.jstor.org/stable/1970438 says for prime modulo there is an algorithm.

Question for non-prime modulo: math.stackexchange.com/questions/4944623/are-diophantine-equations-decidable-in-modular-arithmetic

TODO is it or is not:

- mathoverflow.net/questions/207482/algorithmic-un-solvability-of-diophantine-equations-of-given-degree-with-given
- math.stackexchange.com/questions/181380/second-degree-diophantine-equations
- mathoverflow.net/questions/142938/is-there-an-algorithm-to-solve-quadratic-diophantine-equations
- math.stackexchange.com/questions/798609/is-there-any-solution-to-this-quadratic-diophantine-equation

mathoverflow.net/questions/11540/what-are-the-most-attractive-turing-undecidable-problems-in-mathematics/103415#103415 provides a specific single undecidable Diophantine equation.

mathoverflow.net/questions/11540/what-are-the-most-attractive-turing-undecidable-problems-in-mathematics/11557#11557 contains a good overview of the decidability status of variants over rings other than the integers.

And when it can't, attempt to classify which subset of the integers can be reached. E.g. Legendre's three-square theorem.

4 squares are sufficient by Lagrange's four-square theorem.

3 is not enough by Legendre's three-square theorem.

The subsets reachable with 2 and 3 squares are fully characterized by Legendre's three-square theorem and

Compared to Waring's problem, this is potentially much harder, as we can go infinitely negative in our attempts, there isn't a bound on how many tries we can have for each number.

In other words, it is unlikely to have a Conjecture reduction to a halting problem.

It is exactly what you'd expect from the name, Waring was watching Netflix with Goldbach, when they suddenly came up with this.