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This is a question that I'm trying to solve since yesterday.

Let $T:\mathbb{R}^n\rightarrow\mathbb{R}^n$ be a linear transformation such that

$$\begin{equation} \langle u,v\rangle = 0, \langle Tu,v\rangle >0\quad\Rightarrow\quad \langle u,Tv\rangle >0 \end{equation}$$

We have to proof the following

1) $\langle u,v\rangle = 0$, $\langle Tu,v\rangle =0\quad\Rightarrow\quad \langle u,Tv\rangle =0$;

2) There exists an orthonormal basis for $T$;

3) $T$ is symmetric.

Im stuck in the first item...it really looks easy but i just cant prove this. I used Cauchy-Schwarz inequality to see that if we have $\langle u,v\rangle=0$ and $\langle Tu,v\rangle=0$, then $|\langle u,Tv\rangle + \langle v,Tv\rangle| = 0$, in that case I want to show that $\langle v,Tv\rangle=0$ so I this implies $\langle u,Tv\rangle=0$. Also, with this $u$ and $v$, I have that $\langle u,v+T^\ast v\rangle=0$. I dont know what to do from here...I`m out of ideas, any help is very welcome.

1 Answers 1

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Suppose that $\langle u,v\rangle=0$. Then, of course, $\langle -v,u\rangle=0$.

If $\langle u,Tv\rangle >0$, this is $\langle Tv,u\rangle >0$, and so $\langle v,Tu\rangle >0$, i.e. $\langle Tu,v\rangle >0$.

Similarly, if $\langle u,Tv\rangle <0$, then $\langle u,T(-v)\rangle >0$ and so by the previous paragraph $\langle Tu,-v\rangle >0$, i.e. $\langle Tu,v\rangle <0$.

The last two paragraphs show that if $\langle u,Tv\rangle\ne0$, then $\langle Tu,v\rangle\ne0$. So, if $\langle Tu,v\rangle=0$, then $\langle u,Tv\rangle =0$.

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    it really is $>$2012-12-13
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    Ok, I've modified the answer to use $>$.2012-12-13
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    I dont understand your proof. Remember you dont know yet that T is symmetric. If you are not using this fact, please explain me your argument.2012-12-13
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    I'm not using that $T$ is symmetric. The argument explains itself. If you don't understand some step, tell me and I'll try to explain.2012-12-13
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    ok, thanks! In line 2, how do you go from $\langle Tv, u\rangle >0$ to $\langle v, Tu\rangle >0$ ?2012-12-14
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    I'm applying the hypothesis to the vectors $v,u$.2012-12-14
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    Thank you Martin, you are right.2012-12-14