- #1
AxiomOfChoice
- 533
- 1
If [itex]g(a) \neq 0[/itex] and both [itex]f[/itex] and [itex]g[/itex] are continuous at [itex]a[/itex], then we know the quotient function [itex]f/g[/itex] is continuous at [itex]a[/itex].
Now, suppose we have a linear operator [itex]A(t)[/itex] on a Hilbert space such that the function [itex]\phi(t) = \| A(t) \|[/itex], [itex]\phi: \mathbb R \to [0,\infty)[/itex], is continuous at [itex]a[/itex]. Do we then know that the function [itex]\varphi(t) = \|A(t)^{-1}\|[/itex], [itex]\varphi: \mathbb R \to [0,\infty)[/itex] is continuous at [itex]a[/itex], provided the inverse exists there? Any ideas on how to tackle this question?
I guess I should add that [itex]A(t)[/itex] is a family of bounded linear operators depending on a continuous real parameter [itex]t[/itex].
Now, suppose we have a linear operator [itex]A(t)[/itex] on a Hilbert space such that the function [itex]\phi(t) = \| A(t) \|[/itex], [itex]\phi: \mathbb R \to [0,\infty)[/itex], is continuous at [itex]a[/itex]. Do we then know that the function [itex]\varphi(t) = \|A(t)^{-1}\|[/itex], [itex]\varphi: \mathbb R \to [0,\infty)[/itex] is continuous at [itex]a[/itex], provided the inverse exists there? Any ideas on how to tackle this question?
I guess I should add that [itex]A(t)[/itex] is a family of bounded linear operators depending on a continuous real parameter [itex]t[/itex].
Last edited: