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arxiv: 1512.04421 · v2 · pith:Q72JXD4Enew · submitted 2015-12-14 · 🧮 math.FA

Carl's inequality for quasi-Banach spaces

classification 🧮 math.FA
keywords alphaspacesinequalityquasi-banachcarlgammanumberbanach
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We prove that for any two quasi-Banach spaces $X$ and $Y$ and any $\alpha>0$ there exists a constant $\gamma_\alpha>0$ such that $$ \sup_{1\le k\le n}k^{\alpha}e_k(T)\le \gamma_\alpha \sup_{1\le k\le n} k^\alpha c_k(T) $$ holds for all linear and bounded operators $T:X\to Y$. Here $e_k(T)$ is the $k$-th entropy number of $T$ and $c_k(T)$ is the $k$-th Gelfand number of $T$. For Banach spaces $X$ and $Y$ this inequality is widely used and well-known as Carl's inequality. For general quasi-Banach spaces it is a new result.

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