资料来源 : Free On-Line Dictionary of Computing

surjection
     
         A function f : A -> B is surjective or onto or a
        surjection if f A = B.  I.e. f can return any value in B.
        This means that its {image} is its {codomain}.
     
        Only surjections have {right inverses}, f' : B -> A where
        f (f' x) = x since if f were not a surjection there would be
        elements of B for which f' was not defined.
     
        See also {bijection}, {injection}.
     
        (1995-05-27)