An elementary inductive proof that $AB=I$ implies $BA=I$ for matrices

TL;DR

This paper presents a straightforward, elementary proof that if the product of two square matrices equals the identity, then their reverse product also equals the identity, accessible to first-year students without advanced concepts.

Contribution

It offers a simple, elementary proof of the invertibility implication for matrices, avoiding advanced algebraic tools and suitable for introductory courses.

Findings

Elementary proof of AB=I implies BA=I for matrices

Proof relies solely on basic definitions and elementary operations

Accessible to students in first-year mathematics, physics, or engineering

Abstract

In this note we give an elementary demonstration of the fact that AB=I implies BA=I for square matrices A,B with coefficients in a field K. By elementary we mean that our proof follows from the very definitions of matrix and product of a matrix, with no extra help of more sophisticated results, as the use of dimensions of vector spaces or other ring- theoretical properties, like being Noetherian. The proof is also elementary in the sense that it relies on the concept and properties of the so called elementary operations on matrices. Finally, and no less important, the proof we show can be faced by any good student of a first year course in Mathematics, Physics or Engineering.