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函数f（x）的定义域为D，若存在闭区间[m，n]?D，使得函数f（x）满足：①f（x）在[m，n]上是单调函数；②f（x）在[m，n]上的值域为[2m，2n]，则称区间[m，n]为y=f（x）的“倍值区间”．下列函数中存在“倍值区间”的有______（填上所有正确的序号） ①f（x）=x 2 （x≥0）；②f（x）=e x （x∈R）；③f（x）= 4x x 2 +1 (x≥0) ；④f（x）= log a (a x - 1 8 )(a＞0，a≠1) ．
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