Baby Rudin 1.37—Proof of $|\mathbf x \cdot \mathbf y|\le|\mathbf x||\mathbf y|$

cauchy-schwarz-inequalityinequalitylinear algebrareal-analysisvector-spaces

In Baby Rudin Theorem 1.35, he proves the Schwarz inequality for complex numbers:

If $a_1,\dots,a_n$ and $b_1,\dots,b_n$ are complex numbers, then
$$
\left|\sum_{j=0}^{n}a_j\overline{b_j}\right|^2\le\sum_{j=0}^{n}|a_j|^2\sum_{j=0}^{n}|b_j|^2
$$

Then, in Theorem 1.37, he states the Schwarz inequality for euclidean $k$-spaces:

Suppose $x,y\in\Bbb{R}^k$. Then,
$$
|\mathbf x \cdot \mathbf y|\le|\mathbf x||\mathbf y|
$$

He states that this theorem is an "immediate consequence of the Schwarz inequality" for complex numbers. I don't see how this follows: Theorem 1.35 applies to $\Bbb{C}$, not $\Bbb{R}^k$. What am I missing?

Best Answer

Applying 1.35 with $a_1,\dots,a_n$ and $b_1,\dots,b_n$ being real numbers (which are included in complex numbers), we get $$ \left|\sum_{j=0}^{n}a_j {b_j}\right|^2\le\sum_{j=0}^{n}|a_j|^2\sum_{j=0}^{n}|b_j|^2 .$$

Can you take it from here?

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