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X，Y be sets and f:X-＞Y is a function there's a function g:Y-＞X such that g(f(x))＝x for...

X，Y be sets and f:X-＞Y is a function there's a function g:Y-＞X such that g(f(x))＝x for all x∈X
Prove or disprove: f is a bijection
  
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