1) A is an invetible matrix. 2) A is row equivalent to the nxn identity matrix. 3) A has n pivot positions. 4) The equation Ax=0 has only the trivial solution. 5) The columns of A form a linearly independent set. 6) The linear transformation x I-> Ax is one-to-one. 7) The equation Ax=b has at least one solution for each b in R^n. 8) The columns of A span R^n. 9) The linear transformation x I-> Ax maps R^n onto R^n. 10) There is an nxn matrix C such that CA=I. 11) There is an nxn matrix D such that AD=I. 12) A^T is an invertible matrix.