Examples of theorems that are true conceptually, but false computationally

Here is a longer quote from the article linked in the question:

For most of its history, mathematics was essentially computational: geometry dealt with constructions of geometric objects, algebra was concerned with algorithmic solutions to systems of equations, and analysis provided means to compute the future behavior of systems evolving over time. From the proof of a theorem to the effect that “for every x, there is a y such that …”, it was generally straightforward to extract an algorithm to compute such a y given x.

In the nineteenth century, however, increases in the complexity of mathematical arguments pushed mathematicians to develop new styles of reasoning that suppress algorithmic information and invoke descriptions of mathematical objects that abstract away the details of how those objects are represented. The goal was to obtain a powerful “conceptual” understanding without getting bogged down in computational details, but this had the effect of admitting mathematical theorems that are simply false on a direct computational reading.

With that, we see that the authors are talking about theorems of the form "for every $x$ there is a $y$ ...". There are many such theorems that are true classically, and where objects of the type of $x$ and $y$ could be represented by a computer, but where there is no program to produce $y$ from $x$. This area of study overlaps constructive mathematics and computability theory. In computability theory, rather than just proving that particular theorems are not computably true, we instead try to classify how uncomputable the theorems are. There is also a research field of proof mining which is able to extract algorithms from a number of classical proofs (of course, not all). This program has led to new concrete bounds for theorems in analysis, among other things.

The phenomenon of uncomputability in classical mathematical theorems is very widespread. I will give just a few examples, trying to include several areas of mathematics.

Hilbert's 10th Problem One example comes from Hilbert's 10th problem. Given a multivariable polynomial with integer coefficients, there is a natural number $n$ so that $n = 1$ if the polynomial has an integer root, and $n = 0$ otherwise. This is a trivial classical theorem, but the MDRP theorem shows exactly that there is no program that can produce $n$ from the polynomial in every case.

That is, given a multivariable polynomial with integer coefficients, where we can substitute integers for the variables, there is no effective way to decide whether $0$ is in the range. The proof uses classical computability theory, and shows that this decision problem is equivalent to the halting problem, a benchmark example of an uncomputable decision problem.

Jordan forms Anther example comes when we work with infinite precision real numbers, so that a real number is represented as a Cauchy sequence of rationals that converges at a fixed rate. These sequences can be manipulated by programs, and this is a standard part of computable analysis.

We know from linear algebra that every square matrix over the reals has a Jordan canonical form. However, there is no program that, given a square matrix of infinite precision reals, produces the Jordan canonical form matrix.

The underlying reason for this is continuity: fixing a dimension $N$, the map that takes an $N \times N$ matrix to its Jordan form is not continuous. However, if this function were computable then it would be continuous, giving a contradiction.

Countable graph theory It is easy to represent a countable graph for a computer: we let natural numbers represent the nodes and we provide a function that tells whether there is an edge between any two given nodes. It is a classical theorem that "for each countable graph there is a set of nodes so that the set has exactly one node from each connected component". This is false computationally: there are countable graphs for which the edge relation is computable, but there is no computable set that selects one node from each connected component.

König's Lemma Every infinite, finitely branching tree has an infinite path. However, even if the tree itself is computable, there does not need to be an infinite computable path.

Intermediate value theorem Returning to analysis, we can also represent continuous functions from the reals to the reals in a way that, given an infinite precision real number, a program can compute the value of the function at that number, producing another infinite precision real number. The representation is called a code of the function.

The intermediate value theorem is interesting because it is computably true in a weak sense but not in a stronger sense.

• It is true that, for any computable continuous function $[0,1] \to \mathbb{R}$ with $f(0)\cdot f(1) < 0$, there is a computable real $\xi \in [0,1]$ with $f(\xi) = 0$. So, if we lived in a world where everything was computable, the intermediate value theorem would be true.

• At the same time, there is not a computable functional $G$ that takes as input any code for a continuous function $f$ satisfying the hypotheses above, and produces a $\xi = G(f)$ with $f(\xi) = 0$. So, although the intermediate value theorem might be true, there is no way to effectively find or construct the root just given a code for the continuous function.

This paper by Potgieter has examples to show that, in a suitable computational sense, Brouwer's Fixed point theorem is false. Quoting from the abstract:

The main results, the counter-examples of Orevkov and Baigger, imply that there is no procedure for finding the fixed point in general by giving an example of a computable function which does not fix any computable point.