If in a non-empty partially ordered set $X$ every totally ordered subset (cf. Totally ordered set) has an upper bound, then $X$ contains a maximal element. An element $x_0$ is called an upper bound of a subset $A\subset X$ if $x\leq x_0$ for all $x\in A$. If an upper bound for $A$ exists, then the set $A$ is said to be bounded above. An element $x_0\in X$ is called maximal in $X$ if there is no element $x\in X$, $x\not=x_0$, such that $x_0\leq x$.
|[Ke]||J.L. Kelley, "General topology", Springer (1975) MR0370454|
|[Zo]||M. Zorn, "A remark on a method in transfinite algebra" Bull. Amer. Math. Soc., 41 (1935) pp. 667–670 MR1563165|
Earlier versions of the maximal principle, differing in detail from the one stated above but logically equivalent to it, were introduced independently by several mathematicians, the earliest being F. Hausdorff in 1909. For accounts of the history of the maximal principle, see [Ca]–[RuRu].
|[Ca]||P.J. Campbell, "The origin of "Zorn's lemma" " Historia Math., 5 (1978) pp. 77–89 MR0462876 Zbl 0377.01009|
|[Mo]||G.H. Moore, "Zermelo's axiom of choice", Springer (1982) MR0679315 Zbl 0497.01005|
|[RuRu]||J. Rubin, H. Rubin, "Equivalents of the axiom of choice", 1–2, North-Holland (1963–1985) MR0153590 MR0798475 Zbl 0129.00601|
Zorn lemma. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Zorn_lemma&oldid=32253