New Proofs of Basic Theorems in Calculus

In this note we present new proofs of three basic theorems in calculus. Although these theorems are well-known, in each proof we obtain something which seems to be unknown. We start with the Heine-Cantor theorem about uniform continuity and obtain explicitly the optimal delta for the given epsilon. We then proceed with the Weierstrass extreme value theorem and present two proofs of it: the “envelope proof” in which the largest possible maximal point is found using an envelope function, and the “programmer proof”, which does not use the costume argument of proving boundedness first, and in which an explicit sequence is shown to converge monotonically to the maximal value. We finish with the intermediate value theorem, which is generalized to a class of discontinuous functions and in which the meaning of the intermediate value property is re-examined. In the end we discuss in which sense the proofs are constructive.