The expression 'f(x) = y' denotes a function where 'f' represents the function itself, 'x' is the input value, and 'y' is the output value. In the context of functor composition and identity functors, this notation illustrates how functions map objects and morphisms from one category to another, capturing the essence of relationships in category theory. Understanding this expression helps to comprehend how different categories interact through functors, emphasizing the transformation of structures within mathematical frameworks.
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