1.57: Image
Let X and Y be sets, f be the function f : X → Y , and x be some member of X . Then the image of x under f , denoted f ( x ), is the unique member y of Y that f associates with x .
The image of a subset A ⊆ X under f is the subset of Y defined by
f[ A ] = { y ∈ Y | y = f ( x ) for some x ∈ A }.