discontinuous linear mapping between infinite-dimension vector space

[yifli]

It is known that any linear mapping between two finite dimensional normed vector space is continuous (bounded).

Can anyone give me an example of a linear mapping between two infinite dimensional normed vector space that is discontinuous?

Thanks

[micromass]

Hi yifli! :smile:

It is known that any linear mapping between two finite dimensional normed vector space is continuous (bounded).

Can anyone give me an example of a linear mapping between two infinite dimensional normed vector space that is discontinuous?

Thanks

The standard example is a very familiar linear mapping: differentiation. Let X be the set of all real polynomials on [0,1]. Equip this with the sup-norm, i.e.

\|f\|_\infty=\sup_{t\in [0,1]}{|f(t)|}

Let

T:X\rightarrow X:f\rightarrow f^\prime

Let p_n(x)=x^n, then \|p_n\|_\infty=1, but

\|T(p_n)\|_\infty=n\|p_n\|_\infty

thus the operator T is not bounded.

This is a very tragic result and has a lot of bad consequences...