Re: [複變] 實軸是唯一實值的全純函數至多一零點
保留題目
if f is entire function and f(x) is real if and only if x is real
then f has at most one zero.
可用open mapping principle
設 f 有兩個零點a,b,則 f在開區間 (a,b) 有極大值或極小值 (否則會導致f=0)
可設為f(c)為極小值,但這麼一來,存在c的鄰域U
使 f(c) 屬於 f(U) 交集 R 包含於 [f(c),無限大),f(U) 非開集矛盾
注:由此可見,題目中的entire非必要。
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