intermediate value theorem[edit]
Let f : [a, b] → R be a continuous function, and suppose that f (a) < 0 and f (b) > 0. In this case, the intermediate value theorem states that f must have a root in the interval [a, b]. This theorem can proved by considering the set
S = {s ∈ [a, b] : f (x) < 0 for all x ≤ s} .
That is, S is the initial segment of [a, b] that takes negative values under f. Then b is an upper bound for S, and the least upper bound must be a root of f.