# mathlibdocumentation

linear_algebra.invariant_basis_number

# Invariant basis number property

We say that a ring R satisfies the invariant basis number property if there is a well-defined notion of the rank of a finitely generated free (left) R-module. Since a finitely generated free module with a basis consisting of n elements is linearly equivalent to fin n → R, it is sufficient that (fin n → R) ≃ₗ[R] (fin m → R) implies n = m.

## Main definitions

invariant_basis_number R is a type class stating that R has the invariant basis number property.

## Main results

We show that every nontrivial commutative ring has the invariant basis number property.

## Future work

So far, there is no API at all for the invariant_basis_number class. There are several natural ways to formulate that a module M is finitely generated and free, for example M ≃ₗ[R] (fin n → R), M ≃ₗ[R] (ι → R), where ι is a fintype, or prividing a basis indexed by a finite type. There should be lemmas applying the invariant basis number property to each situation.

The finite version of the invariant basis number property implies the infinite analogue, i.e., that (ι →₀ R) ≃ₗ[R] (ι' →₀ R) implies that cardinal.mk ι = cardinal.mk ι'. This fact (and its variants) should be formalized.

## References

• https://en.wikipedia.org/wiki/Invariant_basis_number

## Tags

free module, rank, invariant basis number, IBN

@[class]
structure invariant_basis_number (R : Type u) [ring R] :
Prop

We say that R has the invariant basis number property if (fin n → R) ≃ₗ[R] (fin m → R) implies n = m. This gives rise to a well-defined notion of rank of a finitely generated free module.

Instances
theorem eq_of_fin_equiv (R : Type u) [ring R] {n m : } :
((fin n → R) ≃ₗ[R] fin m → R)n = m

theorem nontrivial_of_invariant_basis_number (R : Type u) [ring R]  :

theorem invariant_basis_number_field {K : Type u} [field K] :

A field has invariant basis number. This will be superseded below by the fact that any nonzero commutative ring has invariant basis number.

We want to show that nontrivial commutative rings have invariant basis number. The idea is to take a maximal ideal I of R and use an isomorphism R^n ≃ R^m of R modules to produce an isomorphism (R/I)^n ≃ (R/I)^m of R/I-modules, which will imply n = m since R/I is a field and we know that fields have invariant basis number.

We construct the isomorphism in two steps:

1. We construct the ring R^n/I^n, show that it is an R/I-module and show that there is an isomorphism of R/I-modules R^n/I^n ≃ (R/I)^n. This isomorphism is called ideal.pi_quot_equiv and is located in the file ring_theory/ideals.lean.
2. We construct an isomorphism of R/I-modules R^n/I^n ≃ R^m/I^m using the isomorphism R^n ≃ R^m.
@[instance]

Nontrivial commutative rings have the invariant basis number property.