# Cauchy-Schwarz inequality

$LaTeX: |x^T y| \le \|x\| \|y\|,$
where $LaTeX: \|*\|$ is the Euclidean norm. (Note: for $LaTeX: \|x\| = \|y\| = 1$, $LaTeX: x^T y$ is the cosine of the angle between vectors $LaTeX: x$ and $LaTeX: y$.) .