Some Convergence Theorems for Operator Sequences

Let $A,$ $T$ and $B$ be bounded linear operators on a Banach space. This paper is concerned mainly with finding some necessary and sufficient conditions for convergence in operator norm of the sequences $\left\{ A^{n}TB^{n}\right\}$ and \left\{ \frac{1}{n}\sum_{i=0}^{n-1}A^{i}TB^{i}% \right\} . These results are applied to the Toeplitz, composition and model operators. Some related problems are also discussed.