An elementary inductive proof that $AB=I$ implies $BA=I$ for matrices
  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. 
J. M. Almira
math.RA