An actual first-order formulation of ZFC?

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In summary, there are multiple ways of formulating ZFC, but the most commonly used is a first-order axiomatization. This can be found in various resources, such as Wikipedia, and involves replacing higher-order axioms with axiom schema. Another set theory, first-order NBG, is also considered equivalent to ZFC but requires fewer axioms.
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mpitluk
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Can someone point me to a first-order axiomatization of ZFC?

As I've mostly seen ZFC expressed in higher-order logics.
 
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ZFC is a first order theory, where have you been seeing second order formulations? I'm sure every one of the top Google results for "ZFC axioms" will give you a first order formulation. In particular, Wikipedia.
 
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mpitluk said:
Can someone point me to a first-order axiomatization of ZFC?

As I've mostly seen ZFC expressed in higher-order logics.
The higher-order axioms are replaced with axiom schema. e.g. the axiom schema of subsets is collection of statements
{x in A | P(x)} is a set,​
one for every unary predicate P in the language of first-order set theory.First-order ZFC requires infinitely many axioms to specify. First-order NBG, however, is an 'equivalent' set theory in an important sense, but only requires finitely many axioms.
 

1. What is "An actual first-order formulation of ZFC"?

An actual first-order formulation of ZFC refers to a specific way of expressing the Zermelo-Fraenkel set theory with the axiom of choice (ZFC) using only first-order logic. This means that all the axioms and theorems of ZFC can be written in a language with quantifiers (such as "for all" and "there exists") and variables, without the use of set theory or higher-order logic.

2. Why is it important to have a first-order formulation of ZFC?

Having a first-order formulation of ZFC allows for a more precise and rigorous treatment of set theory. It also allows for easier comparisons and connections between different mathematical theories that use first-order logic.

3. How does an actual first-order formulation of ZFC differ from other formulations of ZFC?

An actual first-order formulation of ZFC differs from other formulations of ZFC in that it only uses first-order logic, while other formulations may use set theory or higher-order logic. Additionally, some formulations may have different sets of axioms or different ways of expressing the axioms, but they are all equivalent to each other.

4. What are some applications of an actual first-order formulation of ZFC?

One application of an actual first-order formulation of ZFC is in the foundation of mathematics, as it provides a formal and precise language for discussing sets and their properties. It is also used in many other areas of mathematics, such as mathematical logic, model theory, and proof theory.

5. Are there any limitations to using an actual first-order formulation of ZFC?

While an actual first-order formulation of ZFC is a powerful and widely used tool in mathematics, it does have some limitations. For example, it cannot fully capture the concept of infinite sets, and it is not able to handle certain specialized or non-standard types of sets. Additionally, some mathematicians may prefer to use other formulations of ZFC that better suit their specific needs or interests.

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