- #1
AxiomOfChoice
- 533
- 1
Suppose I have [itex]B: X\to Y[/itex] and [itex]A: Y\to Z[/itex], where [itex]X,Y,Z[/itex] are Banach spaces and [itex]B\in \mathcal L(X,Y)[/itex] and [itex]A\in \mathcal L(Y,Z)[/itex]; that is, both of these operators are bounded. Does it follow that [itex]AB \in \mathcal L(X,Z)[/itex] and
[tex]
\| AB \|_{\mathcal L(X,Z)} \leq \|A\|_{\mathcal L(Y,Z)} \|B\|_{\mathcal L(X,Y)}
[/tex]
It seems like this should be the case, but any time I try to prove a functional analytic result like this, I always get mired in uncertainty about the details...
[tex]
\| AB \|_{\mathcal L(X,Z)} \leq \|A\|_{\mathcal L(Y,Z)} \|B\|_{\mathcal L(X,Y)}
[/tex]
It seems like this should be the case, but any time I try to prove a functional analytic result like this, I always get mired in uncertainty about the details...