Monday, March 26, 2018

Goodstein's Theorem and Peano Arithmetic

This is the second part of a post series on Goodstein's theorem. For the first part, see here.

We saw last post that Goodstein sequences are certain sequences of positive integers defined using base representations. Despite their simple definition, they grow extraordinarily large. However, Goodstein's Theorem states that no matter how large the starting value is, the sequence will eventually terminate at 0 after some finite number of steps. Before discussing the proof to this remarkable theorem, we move in a completely different direction and define the axioms of Peano arithmetic.

Increasing standards of rigor were a hallmark of late 19th and early 20th century mathematics. With this movement came a need to precisely define the basic building blocks with which a given branch of mathematics worked. This was even true for simple arithmetic! By 1890, the mathematician Giuseppe Peano had published a formulation of the axioms (fundamental assumptions) of arithmetic, which are used nearly unchanged to this day. Written in simple english, the axioms state the following about the collection N of natural numbers (nonnegative integers), a function S known as the successor function, a function + known as addition, and a function * known as multiplication:

1) There exists a natural number called 0 in N
2) For every natural number n in N there is a successor S(n) in N (commonly written
n + 1)
3) There is no natural number whose successor is 0
4) For any two natural numbers m and n in N, if S(m) = S(n), then m = n
5) For any natural number n, n + 0 = n
6) For any natural numbers n and m, n + S(m) = S(m + n)
7) For any natural number n, n*0 = 0
8) For any natural numbers n and m, n*S(m) = n*m + n
9) For any "well-behaved" property P, if P(0) holds and for each n in N, P(n) implies
P(n + 1), then P is true for every natural number in N

The first two axioms simply say that you can start from 0 and count upwards forever through the natural numbers. The third says that there is no natural number below 0 (this is of course false for larger sets of numbers such as integers, but Peano's axioms are only for properties of the natural numbers). The fourth shows that the successor of a natural number is unique. The fifth through eighth axioms state the common properties of addition and multiplication. The ninth axiom is actually a large collection of axioms (an axiom schema) in disguise, called the "axiom schema of induction."

The idea of induction may be familiar to readers who recall their high school mathematics: one often wishes to prove that a given property holds for every natural number. To do so, it is sufficient to show that it is true in the "base case," that is, true for 0, and that the statement being true for a number means that it is also true for the next. Then since the property is true for 0, it is true for 1. Since it is true for 1, it is true for 2, and so on. The figure above illustrates the idea of induction, where each "statement" is that the given property is true for some number. The final axiom simply codifies the fact this type of reasoning works; we get that the property is true for all natural numbers. In this context, a "well-behaved" property is one expressible in the semantics of the variety of logic being used (first-order logic in this case).

Remarkably, the above assumptions are all that is needed to perform arithmetic. In principle, though formalizing complicated proofs would be quite lengthy, true statements such as "17 is prime" and "every number is the sum of four squares" become theorems in Peano arithmetic. All of these proofs would take the form of a chain of statements beginning from axioms and concluding with the desired result, such as "17 is prime," rendered appropriately in the formal mathematical language. The progression from each statement to the next would also follow a collection of well-defined deductive rules. In principle, almost all theorems concerning natural numbers could be proven this way, beginning from just a small collection of axioms! However, Peano arithmetic still runs afoul of a result that vexes many axiom systems of first-order logic: Gödel's Incompleteness Theorem.

Gödel's (First) Incompleteness Theorem, originally proven by the mathematician Kurt Gödel in 1931, dashed the hopes of those who imagined that formal logical systems would provide a complete description of mathematics. It states, informally, that for any first-order logical system powerful enough to encode arithmetic (Peano arithmetic is of course such a system), there exist statements in the language of the system that are neither provable nor refutable from the axioms. Further, there are explicit sentences in the logical system that are true (under the intended interpretation of the theory, more on this later) but unprovable. The above diagram illustrates the situation: there will always be things we know to be true or false that are beyond the reach of the axioms to formally prove or refute.

Some questions may spring to mind at this unintuitive result. What is the distinction between "true" and "provable"? How do we define "true" in mathematics if not as the end result of a proof? What do these unprovable statements look like, and what do they say?

The answer to the first of these depends on there being something "we mean by" the term "natural numbers". In other words, there is an intended interpretation of what natural numbers should be that the logical system fails to completely capture. Consequently, there are statements that we know to be true using methods outside the formal system but are unprovable within it. Bringing in additional assumptions does not simply resolve the incompleteness theorem, however. For each outside axiom added, the theorem guarantees the existence of a new unprovable statement. And if the system ever does become complete by the addition of more axioms, it also becomes inconsistent, that is, able to prove a contradiction (and loses all validity as a mathematical system).

As for the final question, the first known unprovable statements were those constructed in the proof of Gödel's theorem; these are known as Gödel sentences. They are highly contrived for the proof, however, and do not have any intuitive meaning. In the years following the original proof, the concern remained whether, for example, any statement that "naturally" arises in the study of natural numbers would be unprovable and unrefutable from the axioms of Peano arithmetic. Amazingly, such statements exist! In fact, a great example is Goodstein's Theorem. No proof exists, beginning from the Peano axioms, that has it as a conclusion. To read more about how it can be proven and why it is not a theorem of Peano arithmetic, see the next post.

Sources: https://www.cs.toronto.edu/~sacook/csc438h/notes/page96.pdf, https://plato.stanford.edu/entries/goedel-incompleteness/