By Theorem 2.7.9, between any two real numbers, points on the number line, there exists a rational number. This suggests there are a lot of rational numbers, but it is not clear from this Theorem whether the entire real line consists of only rational numbers. Some people might wish this were the case because then each real number could be described, not just as a point on a line but also algebraically, as the quotient of integers. Before 500 B.C., a group of mathematicians, led by Pythagoras believed in this, but they discovered their beliefs were false. It happened roughly like this. They knew they could construct the square root of two as the diagonal of a right triangle in which the two sides have unit length; thus they could regard
This shows that if it is desired to consider all points on the number line, it is necessary to abandon the attempt to describe arbitrary real numbers in a purely algebraic manner using only the integers. Some might desire to throw out all the irrational numbers, and considering only the rational numbers, confine their attention to algebra, but this is not the approach to be followed here because it will effectively eliminate every major theorem of calculus. In this book real numbers will continue to be the points on the number line, a line which has no holes. This lack of holes is more precisely described in the following way.
Definition 2.10.1 A non empty set, S ⊆ ℝ is bounded above (below) if there exists x ∈ ℝ such that x ≥
Every existence theorem in calculus depends on some form of the completeness axiom. In an appendix, there is a proof that the real numers can be obtained as equivalence classes of Cauchy sequences of rational numbers.
Axiom 2.10.2 (completeness) Every nonempty set of real numbers which is bounded above has a least upper bound and every nonempty set of real numbers which is bounded below has a greatest lower bound.
It is this axiom which distinguishes Calculus from Algebra. A fundamental result about sup and inf is the following.
Proposition 2.10.3 Let S be a nonempty set and suppose sup

If inf

Proof:Consider the first claim. If the indicated set equals ∅, then sup