Let f be a function defined on [0, 1]with the following property:
For every real number y,
either there is no x in [0, 1] for which f(x)=y
or there are exactly two values of x in [0, 1] for which f(x)= y.
Prove that any function with this property has infinite many
discontinuities on [0, 1].
希望可以給予指點，謝謝^^
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