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### B.3. Norms

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The norm is something that comes up often in machine learning literature. The norm of a matrix is written with two vertical lines on each side of the matrix like this: ||A||. Let’s talk about vector norms first.

The vector norm is an operation that assigns a positive scalar value to any vector. You can think of this as the length of the vector, which is useful in many machine learning algorithms such as k-Nearest Neighbors. If you have a vector `z=[3,4]` the length is. This is also known as the L2 norm and is written as ||z|| or ||z||^{2}.