L20A
       第二次期中考第1,2,3,4,5,7題
       這裡U代表複平面上的單位開圓盤
       第1題: Find holomorphic functions whose real parts is x^3y-xy^3.
       第2題: Prove that every nonconstant holomorphic function is an open mapping.
       第3題: Suppose λ>1. Prove that λ-z-exp(-z) has exactly one root in the right half plane.
       第4題: Let f be holomorphic on U\\\\\\\\{0} and 0