Function
A rule that takes an input x and produces a single output f(x).
Definition
Function notation f(x) replaces y. f(2) means substitute 2 for x. Composite functions like fg(x) mean apply g first, then f to the result. Inverse functions f⁻¹(x) reverse f.
Example
If f(x) = 3x + 1 then f(2) = 7. Inverse: f⁻¹(x) = (x − 1)/3.
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