# On some inequalities for the approximation numbers of the sum and product of operators

## Authors

• Nicolae Tiţa “Transilvania” University of Brasov, Romania

## Keywords:

approximation numbers, symmetric norming function
Abstract views: 190

## Abstract

We prove the inequalities:$$\textstyle\sum\limits_{n=1}^{k} a_{n} \left(\textstyle\sum\limits_{i=1}^{r} S_{i}\right) \le r\textstyle\sum\limits_{n=1} ^{k}\, \textstyle\sum\limits_{i=1}^{r}a_{n}(S_{i}),$$$$\textstyle\sum\limits_{n=1}^{k} a_{n}\left(\textstyle\prod\limits_{i=1}^{r} S_{i}\right) \leq r\textstyle\sum\limits_{n=1}^{k} \textstyle\prod\limits_{i=1}^{r}a_{n}(S_{i})\;,\;k=1,2,...,\;\;r\geq2,$$and$$\textstyle\prod\limits_{n=1}^{k} a_{n}\left(\textstyle\prod\limits_{i=1}^{r} S_{i}\right)\leq\textstyle\prod\limits_{n=1}^{k}\textstyle\prod\limits_{i=1}^{r}a_{n}^{r}(S_{i})\;,\;k=1,2,...,\;\;\;r \geq2,$$where $$\left\{ a_{n}(S)\right\} \;$$ is the sequence of the approximation numbers of the linear and bounded operators $$S: X\rightarrow X$$ $$(S\in L(X))$$. $$X$$ is a Banach space.

## References

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2005-02-01

## How to Cite

Tiţa, N. (2005). On some inequalities for the approximation numbers of the sum and product of operators. Rev. Anal. Numér. Théor. Approx., 34(1), 109–113. https://doi.org/10.33993/jnaat341-797

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