p范数（p norm）

先回顾一下范数的定义（en.wikipedia.org/wiki/Norm_(mathematics)）：

Given a vector space V over a subfield F of the complex numbers, a norm on V is a function p: V → R with the following properties:[1]

For all a ∈ F and all u, v ∈ V,

By the first axiom, absolute homogeneity, we have p(0) = 0 and p(-v) = p(v), so that by the triangle inequality

经常会听到p范数（p norm）的说法，其实很简单，可以看成2范数的扩展，但是有一点需要注意：p的范围是[1, inf)。p在(0,1)范围内定义的并不是范数，因为违反了三角不等式（||x+y||