Prove that a normed space is not Banach

1. Feb 16, 2012

russel

Hello everyone,
I have a problem and cannot solve it. Could you help? Here it is
We have a normed space and an uncountable Hamel basis of it. Prove that it is not a Banach space.
Should I use Baire theorem? Any suggestions?

2. Feb 16, 2012

micromass

You're not giving us much information to work with. If you want to show something not Banach, then try to find a Cauchy sequence which does not converge.
There are other ways to prove this of course. A lot depends on the space in question.

3. Feb 16, 2012

alan2

You must have a particular space in mind that you didn't specify. An infinite dimensional Banach space necessarily has an uncountable Hamel basis so your result is not general. You need to show that your space is not complete. What is your space?

4. Feb 16, 2012

russel

The exercise does not refer to a particular space. It is just a normed space X with an uncountable Hamel basis. A solution I came up with was to make a finite dimension closed subspace and show using baire that it is X, leading to a contadiction.
If the problem gave us a space for this example which one would that be?

5. Feb 16, 2012

micromass

Could you quote the exercise exactly as it was given??

6. Feb 16, 2012

Bacle2

But, how could you prove that a closed subspace of an undefined space equals the space itself?

7. Feb 16, 2012

micromass

Also notice that a finite dimensional normed space is always Banach, so your proof is likely incorrect.