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Together with the axiom of choice (see below), these are the de facto standard axioms for contemporary mathematics or set theory. They can be easily adapted to analogous theories, such as mereology. Axiom of extensionality; Axiom of empty set; Axiom of pairing; Axiom of union; Axiom of infinity; Axiom schema of replacement; Axiom of power set ...
The following particular axiom set is from Kunen (1980). The axioms in order below are expressed in a mixture of first order logic and high-level abbreviations. Axioms 1–8 form ZF, while the axiom 9 turns ZF into ZFC. Following Kunen (1980), we use the equivalent well-ordering theorem in place of the axiom of choice for axiom 9.
Set theory is also a promising foundational system for much of mathematics. Since the publication of the first volume of Principia Mathematica, it has been claimed that most (or even all) mathematical theorems can be derived using an aptly designed set of axioms for set theory, augmented with many definitions, using first or second-order logic.
Many different equivalent complete axiom systems have been formulated. They differ in the choice of basic connectives used, which in all cases have to be functionally complete (i.e. able to express by composition all n-ary truth tables), and in the exact complete choice of axioms over the chosen basis of connectives.
The first of these, called the theory of true arithmetic, cannot be written as the set of logical consequences of any enumerable set of axioms. The theory of ( R , +, ×, 0, 1, =) was shown by Tarski to be decidable ; it is the theory of real closed fields (see Decidability of first-order theories of the real numbers for more).
There are many other equivalent statements of the axiom of choice. These are equivalent in the sense that, in the presence of other basic axioms of set theory, they imply the axiom of choice and are implied by it. One variation avoids the use of choice functions by, in effect, replacing each choice function with its range:
In many popular versions of axiomatic set theory, the axiom schema of specification, [1] also known as the axiom schema of separation (Aussonderungsaxiom), [2] subset axiom [3], axiom of class construction, [4] or axiom schema of restricted comprehension is an axiom schema. Essentially, it says that any definable subclass of a set is a set.
Using first-order logic primitive symbols, the axiom can be expressed as follows: [2] ( ( ()) ( ( (( =))))). In English, this sentence means: "there exists a set 𝐈 such that the empty set is an element of it, and for every element of 𝐈, there exists an element of 𝐈 such that is an element of , the elements of are also elements of , and nothing else is an element of ."