Some new bounds on the entropy numbers of diagonal operators

摘要

Entropy numbers are an important tool for quantifying the compactness of operators. Besides establishing new upper bounds on the entropy numbers of diagonal operators D-sigma from l(p) to l(q), where p not equal q, we investigate the optimality of these bounds. In the case of p < q optimality is proven for fast decaying diagonal sequences, which include exponentially decreasing sequences. In the case of p > q we show optimality under weaker assumption than previously used in the literature. In addition, we illustrate the benefit of our results with examples not covered in the literature so far. (C) 2019 Elsevier Inc. All rights reserved.