# Math Help - Hahn-Banach Theorem (Normed Spaces)

1. ## Hahn-Banach Theorem (Normed Spaces)

Dear Colleagues,

I have a problem in the attachment and could you please help me in solving it.

Best Regards.

Raed.

2. Since $\mathbb{R}^n$ is an inner-product space, every linear functional is given by an inner product. In fact, if $\alpha = (\alpha_1,\alpha_2)$ then the functional $f(x) = \alpha_1\xi_1+\alpha_2\xi_2$ is given by $f(x) = \langle x,\alpha\rangle$. Use that fact to show that $\|f\|_{\mathbb{R}^2} = \sqrt{\alpha_1^2+\alpha_2^2}$.

Now that you know $\|f\|$, you have to figure out how to extend f to a linear functional on $\mathbb{R}^3$ without increasing the norm.

3. Thank you very much for your reply I have already proved the norm of f. But the problem is in finding a linear extension of f that preserves the norm.

Best Regards.

4. Originally Posted by raed
I have already proved the norm of f. But the problem is in finding a linear extension of f that preserves the norm.
So the extension to $\mathbb{R}^3$ must be of the form $\tilde{f}(x) = \alpha_1\xi_1+\alpha_2\xi_2+\alpha_3\xi_3$, and its norm will be $\|\tilde{f}\| = \sqrt{\alpha_1^2+\alpha_2^2+\alpha_3^2}.$ That leaves only one choice for $\alpha_3.$