Department of Mathematical Sciences
Piazzale Europa, 1
34127 Trieste Italy
We discuss the problem of stability in difference equations, which are written in the form Yn=(A0...An-1)Y0. When An= A then the stability is ensured if the spectral radius of A is strictly less than 1. In the case of variable coefficients the problem is by far more complicated. Then we need to introduce the notion of joint spectral radius.