[math] max and argmax

출처 [https://math.stackexchange.com/questions/114260/what-is-the-right-symbol-for-the-max-like-pi-is-for-product]

[https://en.wikipedia.org/wiki/Arg_max]

max_{x\in\mathcal{R}} f(x): maximum value of f(x) when x is real number

argmax_{x\in\mathcal{R}} f(x): x which gives manimum f(x)