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When I am reading a machine learning book I came across the following notation


What does the $||.||^2_2$ stand for? I know its magnitude but what is the 2,2 for?


merged by whuber Dec 15 '15 at 19:17

This question was merged with What is the meaning of super script 2 subscript 2 within the context of norms? because it is an exact duplicate of that question.