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Let $B_1,B_2$ be two Banach spaces and $L(B_i,B_j),K(B_i,B_j)(i,j=1,2)$ spaces of bounded and compact linear operator between them respectively. If $T \in L(B_1,B_1)$, we have a $S \in K(B_1,B_2)$ and a constant $c>0$ such that for any $v \in B_1$,$${\left\| {Tv} \right\|_{{B_1}}} \le c{\left\| v \right\|_{{B_1}}} + {\left\| {Sv} \right\|_{{B_2}}}.$$

My question is, can we find a $A \in K(B_1,B_1)$, such that ${\left\| {T - A} \right\|_{L({B_1},{B_1})}} \le c$?

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    The case when finite-rank operators are dense in $K(B_1,B_2)$ should be easier; do you have a proof in this case?2012-12-30
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    What if $T$ is the identity operator?2013-01-09
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    @RobertIsrael If $T$ is the identity, then the problem reduces to showing that the displayed inequality forces $c\ge 1$. The latter is easy to see when the pair $(B_1,B_2)$ has the approximation property (hence my previous comment), but perhaps not as easy in general.2013-01-15
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    @RobertIsrael If $T$ is the identity, then $c \le 1 - \varepsilon$ would imply $\|Sv\|_{B_2} \ge \varepsilon \|v\|$, rendering $S$ invertible and both $B_1$ and $B_2$ finite-dimensional. So for infinite-dimensional $B_1$ or $B_2$, we must have $c \ge 1$ and one can choose $A = 0$.2016-12-24

1 Answers 1

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To start from a very simple case: If $B_1$ is a Hilbert space and $S$ is finite-dimensional such that we have $$ \|Tv\| \le c\|v\| + \|Sv\| \quad \forall v$$ then we can find a finite-dimensional $A$ satisfying $$\tag{1} \|Av\| \le \|Sv\|$$ and $$\tag{2} \|(T-A)v\| \le c \|v\|$$ for every $v \in B_1$.

Proof: Write $B_1 = (\ker S)^\bot \oplus_2 \ker S$. We define $A$ separately on each summand: On $\ker S$, we can clearly choose $A = 0$. On its annihilator, we can work with an orthonormal basis: For each vector $e_n$, we set $$ Ae_n = \frac{\|Se_n\|}{c + \|Se_n\|} Te_n$$ (note that we know $Se_n \ne 0$) which immediately gives us $$ \|Ae_n\| \le \|Se_n\|$$ and $$ \|(T-A)e_n\| = \left\|\left(1 - \frac{\|Se_n\|}{c + \|Se_n\|} \right)Te_n\right\| \le c $$ Since (1) and (2) are thus satisfied both on all of $\ker S$ and whenever $v$ is member of the orthonormal basis for $(\ker S)^\bot$, i.e. $v = e_n$, they are satisfied for every $v \in B_1$.