Saturday, 29 June 2019

limits - Sequence of norms

Suppose that we have a sequence of norms ( on a finite dimensional space X such that, as functions from X to \mathbb{R}_+, they point-wise converge to a limit norm \|\cdot\|_\infty. Since we are in a finite-dimensional setting, all of the norms are equivalent among them, and, in particular, all of the norms in the sequence are equivalent to \|\cdot\|_\infty. An implication of this fact is that there is a sequence of constants c_n such that
\forall n \geq 1, \forall x\in X, \quad \|x\|_n \geq c_n\|x\|_\infty.



Now, the equivalence constants c_n are not necessarily unique, but, since the norms \|\cdot\|_n approach \|\cdot\|_\infty in the limit, I am wondering how can one prove that they can be chosen to have 1 as a limit (i.e. \lim_{n\to\infty} c_n = 1) ?

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