A function f: A → B is a relation that pairs every input with exactly one output — no input skipped, none mapped twice. The allowed inputs form its domain; the values it actually produces form its range. When both live inside the reals ℝ, it's a real function — the workhorse of all calculus.
- Image / preimage — if f(a) = b, then b is the image of a, and a a preimage of b.
- Domain rule — every element of A must be paired; the domain is all of A.
- Range ⊆ codomain — f(x) = x² maps ℝ → ℝ but only reaches [0, ∞).



