ProvenMath > Set Theory > Axioms of Set Theory
This page states the axioms of set theory in a formal and precise manner in terms of quantifiers, logical operators, and the membership symbol. The use of the English language (including the word 'set') is very careful and consistent resulting in a text that may perhaps be parsed and analyzed by a computer program. All objects are called 'sets' even when we think of them as elements. Thus the elements of a set are also sets.
The axiom which states that for any set X and any formula P there exists a subset of X containing exactly the elements x for which P(x) is true is stated very precisely using notions like
The Axiom of Choice is given on a separate page where it is shown to be equivalent with the Well-Ordering Principle, Hausdorff's Maximal Principle, and Zorn's Lemma. Strict proofs are given for the equivalence of the Axiom of Choice with any of the above-mentioned principles.
ProvenMath > Set Theory > Operations on Sets
This page introduces basic operations on sets like
ProvenMath > Set Theory > Cartesian Product, Relation and Function
The following concepts are introduced on this page:
ordered pair (a,b), power set, Cartesian product, relation, equivalence relation,
function, 1-1 function, "onto" function, composition of functions fog,
the inverse function
ProvenMath > Set Theory > Infinite Union and Intersection
This page states and proves the De Morgan's Laws for infinite union and intersection of sets.
ProvenMath > Set Theory > Orders
The following concepts are introduced on this page: partial order, linear order, and well-order.
ProvenMath > Set Theory > Induction and Recursion
This page states and proves the Induction Principle and the Recursion Principle for any well-ordered set providing the framework for recursive definitions. We use these principles to prove that the Well-Ordering Principle implies Hausdorff's Maximal Principle.
ProvenMath > Set Theory > Axiom of Choice and Its Equivalents
This page states the Axiom of Choice, the Well-Ordering Principle, Hausdorff's Maximal Principle, and Zorn's Lemma, and proves that they are equivalent under the first six axioms - without the axiom of infinity and - more importantly - without the axiom of regularity.
For those professional mathematicians who have never found the time to go through the equivalence proofs this page can serve as a reassurance: it's all here on a single page and can be gone through at any time given a spare evening or two.
ProvenMath > Set Theory > Cardinality - without Axiom ZF9
This page introduces the following cardinality comparisons and proves the basic theorems about these concepts.
||X| = |Y|||X and Y have the same cardinality|
||X| <= |Y|||Y is at least as numerous as X|
||X| < |Y|||Y is strictly more numerous than X|
Two theorems deserve special attention:
|1|||X| < |P(X)||
|2|||X| <= |Y| or |Y| <= |X||
The first theorem shows that we can create sets of ever greater cardinality.
And what's worth noticing at this point is that we don't need the Axiom of Choice
to assert that the power set
ProvenMath > Set Theory > Ordinals and Cardinals - with Axiom ZF9
This page defines the concept of an ordinal and proves many basic theorems about ordinals using heavily the Axiom of Regularity (ZF9). The Axiom of Choice is not used in these proofs.
ProvenMath > Set Theory > Natural Infinite Set
This page proves the existence of a unique minimal set M containing the empty set
and having the following transitivity property:
We call this set the Natural Infinite Set, and we use it to define the notion of countability and the destinction between finite and infinite sets.
To obtain this set we use the Axiom of Infinity and this is the first and only time when this axiom is used. We don't need the Axiom of Choice or the Axiom of Regularity to obtain this set but we use the Axiom of Regularity to show that the Natural Infinite Set is an ordinal and that all of its elements are ordinals.
ProvenMath > Topology > Topological Space
This page defines the following concepts: topological space, open set, closed set, interior, closure,
and states the most basic theorems about these concepts with complete elementary proofs.
For example, it contains basic facts like
ProvenMath > Topology > Subspace of Topological Space
This page defines the concept of a topological subspace and provides all the basic theorems for dealing with subspaces. Interestingly, it is quite easy to convince oneself that the definition of the relative topology satisfies the axioms for a topological space. However, the easy straightforward proof uses the Axiom of Choice in a way that is almost transparent to an unexperienced eye. This page proves this simple well-known fact in a slightly long and meticulous way in order to avoid the use of the Axiom of Choice.
ProvenMath > Topology > Separation Axioms
The page contains a very precise formulation of the following topological concepts: T_0 space, T_1 space, T_2 space, T_3 space, T_4 space, T_5 space, Hausdorff space, regular space, and normal space. It includes basic theorems showing appropriate implications between these separation properties. For more detailed information about separation conditions see the recommended book Counterexamples in Topology by Lynn Arthur Steen and J. Arthur Seebach, Jr. - which lists all separation conditions (including T_2.5, T_3.5, semiregular, completely Hausdorff, perfectly normal, etc.) and shows all implications that hold between them and gives counterexamples for the implications that do not hold in general.