以下は実数の完備性を特徴付ける同値な命題であり, 同じく順序体である有理数では成り立たない。
Axiom of Completeness Every nonempty set of real numbers that is bounded above has a least upper bound. Th. 上限(下限)とは,上界(下界)側からちょっとでも踏み込むと超えてしまうギリギリライン。 ⇔ 1. 2.
Th. Nested Interval Property For each n∈N, assume we are given a closed interval In:=[an,bn]. Assume also that each In contains In+1.Then, the resulting nested sequence of closed intervals has a nonempty intersection; that is,
Th. Monotone Convergence Theorem If a sequence is monotone and bounded, then it converges.
Th. Bolzano-Weierstrass Every bounded sequence contains a convegent subsequence.
Th. Cauchy Criterion A sequence converges if and only if it is a Cauchy sequence.