olemus
ruutmaatriks A, millele on olemas pöördmaatriks B,
st kehtib AB = BA = I , kus I on ühikmaatriks

Wiktionary:
any n×n square matrix for which there exists a corresponding inverse matrix (i.e., a second (or possibly the same) matrix such that when the two are multiplied by each other, in either order, the result is the n×n identity matrix)

ülevaateid
http://www.sosmath.com/matrix/matinv/matinv.html

https://www.saylor.org/site/wp-content/uploads/2011/05/Invertible-Matrix.pdf

https://en.wikipedia.org/wiki/Invertible_matrix