# nLab finite mathematics

Finite mathematics

# Finite mathematics

## Idea

Finite mathematics is the mathematics of finite sets. The term is sometimes used more broadly for discrete mathematics.

We may say that finite mathematics is mathematics done internal to the category FinSet of finite sets or the mathematics of $\Fin\Set$ itself. The latter (but not the former) includes the basic arithmetic of natural numbers, since these are the cardinalities of finite sets; we can go as far as rational numbers this way, but not real numbers.

Finite mathematics also includes a great deal of combinatorics, basic algebra, and elementary formal logic, although not many advanced topics.

## Finitism

In the foundations of mathematics and in philosophy of mathematics, finitism is the philosophical sentiment that one “should” do only finite mathematics. In a weak sense, one should not assume the axiom of infinity; in a strong sense, one should even deny it by an axiom of finiteness. This makes it impossible to do analysis as we normally understand it.

Finitism (in the weak sense of not accepting an axiom of infinity) is essentially the mathematics that can be done internal to an arbitrary boolean topos (at least if one is not also being predicative or constructive). For constructive mathematics as usually practised, one goes beyond finitism by positing a natural numbers object.

Although often considered an extreme form of constructivism, finitism in the strong sense (actually denying the axiom of infinity) can make excluded middle and even the axiom of choice constructively acceptable (and similarly make power sets predicatively acceptable). This is because even constructivists agree that these are true in $\Fin\Set$; it's the extension of them to infinite sets that the first constructivists objected to.

Ultrafinitism is an even more extreme form of finitism, in which one doubts the existence of certain very large natural numbers. The theory of ultrafinite mathematics is most well developed by Edward Nelson in Nelson arithmetic. Foundational systems such as soft linear logic can also be argued to have an ultrafinitist flavor.

This is also true of foundational systems using paraconsistent logic, where paraconsistent arithmetics can have only a finite number of natural numbers. In particular, there is a paraconsistent version of the Löwenheim-Skolem theorem, which states:

Any mathematical theory presented in first order logic has a finite paraconsistent model.

For the opinionated espousal of finitism (and much else), one can hardly do better than the Opinions of Doron Zeilberger.

### The mathematics of FinSet

One definition of finite mathematics is as the mathematics of FinSet. This includes the basic arithmetic of natural numbers, since these are the cardinalities of finite sets. This may seem contradictory to the entire conception of “finite mathematics”, since the natural numbers form an infinite set. The way out is to state that the natural numbers don’t form a set (or class) at all; instead, they are formally defined outside of the set theory via an auxiliary theory like primitive recursive arithmetic. This phenomenon is similar to defining the universe hierarchy of Russell universes or Coquand universes in dependent type theory without a separate type judgment, where the natural numbers used in the universe indices are not elements of a type, but rather formally defined outside of the type theory using some other theory.

### Neutral finite mathematics

An alternative definition of finite mathematics is neutral finite mathematics; i.e. mathematics done internally to an elementary topos (constructively) or Boolean topos (classically), but which does not assume either the axiom of infinity or the axiom of finiteness. There are no natural numbers; instead one works directly with the finite sets in neutral finite mathematics in neutral finite mathematics.

Alternatively, one can attempt to approximate arithmetic in the natural numbers by considering increasing long tuples of digits - elements of a finite set $D$, via the inclusions

$\mathbb{1} \hookrightarrow D \hookrightarrow D \times D \hookrightarrow D \times D \times D \hookrightarrow \ldots$

In particular, in the internal type theory of neutral finite mathematics, since types in the internal type theory aren’t free variables, one cannot quantify over finite sets, which means that many concepts in elementary number theory cannot be internally defined in neutral finite mathematics for finite sets. Examples include:

• Unlike the case for division of natural numbers, which can be defined via induction or recursion on the natural numbers, the division of two finite sets cannot be defined via the usual set-theoretic operations on finite sets in neutral finite mathematics. Instead, one needs to add additional set constructors to neutral finite mathematics which state that given finite set $A$ and finite pointed set $B$ with an element $p:B$, one can construct finite sets $A \div B$ and $A\ \%\ B$ with bijections

$A \simeq ((B \times (A \div B)) + (A\ \%\ B)) \qquad (B \hookrightarrow A\ \%\ B) \simeq \emptyset$

meaning that it is no longer neutral finite mathematics.

• As a result, the analogues of the notions of divisibility relation, divisor, greatest common divisor, least common multiple, prime number, prime factorization, and so forth, cannot be internally defined for finite sets in neutral finite mathematics.

• Furthermore, the definition of prime number requires quantification over finite sets, which is impossible in the internal logic since there are no finite set free variables. (The collection of finite sets are infinite, and so internally quantifying over that collection implies that one has an infinite collection in the foundations, contrary to finitism.) From this internal perspective, Doron Zeilberger, despite his Opinions, is not finitist since he is able to define prime numbers and work with them (cf. Zeilberger 01), which requires some notion of quantification over finite sets to define finite sets with prime number cardinality, or otherwise the infinite set of natural numbers so that one can internally quantify over them for the definition of prime number.

• One can prove that given individual finite sets $A$ and $B$ that there is a bijection $A + B \simeq B + A$, but one cannot prove that there is a bijection $A + B \simeq B + A$ for all $A$ and $B$, because there are no finite set free variables in the internal logic. The same goes for any arithmetic or order-theoretic property for the set-theoretic operations on finite sets. Syntactically, the properties for finite sets are given by inference rules with at least one hypothesis, rather than axioms as would be for the natural numbers:

$\frac{A \; \mathrm{set} \quad p:\mathrm{isFinite}(A) \quad B \; \mathrm{set} \quad q:\mathrm{isFinite}(A)}{\mathrm{comm}_{A + B}(p, q):A + B \simeq B + A}$
• In predicative constructive neutral finite mathematics; i.e. internal to a $\Pi$-pretopos without the natural numbers or a set of truth values, one can’t even define when a set is a finite set, and thus one cannot do any natural numbers arithmetic in the guise of finite set arithmetic at all. Even basic arithmetic properties of the natural numbers like the cancellativity of multiplication don’t apply to arbitrary sets in the absence of excluded middle (cf. Swan 18), and arbitrary sets only form a preorder, rather than a total preorder.

### $\pi$-finite mathematics

In homotopy type theory, another possibility is available: $\pi$-finite mathematics, which can be said to be the study of only the $n$-truncated pi-finite types for $n:\mathbb{N}$, which have finite homotopy groups, and type operations that preserve finiteness. Finite sets are just finite 0-truncated tame homotopy types in homotopy type theory. Certain type operations do not preserve tameness, such as suspensions and homotopy pushouts, and so wouldn’t be part of finite mathematics. This extends traditional finite mathematics based in set theoretic foundations to more modern fields such as higher category theory and homotopy theory. Just as the cardinalities of finite sets are natural numbers (finite set cardinalities), the cardinalities of these $n$-truncated homotopy types are positive rational numbers.