To install click the Add extension button. That's it.

The source code for the WIKI 2 extension is being checked by specialists of the Mozilla Foundation, Google, and Apple. You could also do it yourself at any point in time.

4,5
Kelly Slayton
Congratulations on this excellent venture… what a great idea!
Alexander Grigorievskiy
I use WIKI 2 every day and almost forgot how the original Wikipedia looks like.
Live Statistics
English Articles
Improved in 24 Hours
Languages
Recent
Show all languages
What we do. Every page goes through several hundred of perfecting techniques; in live mode. Quite the same Wikipedia. Just better.
.
Leo
Newton
Brights
Milds

In mathematical set theory, the axiom of adjunction states that for any two sets x, y there is a set w = x ∪ {y} given by "adjoining" the set y to the set x. It is stated as

${\displaystyle \forall x.\forall y.\exists w.\forall z.{\big (}z\in w\leftrightarrow (z\in x\lor z=y){\big )}.}$

Bernays (1937, page 68, axiom II (2)) introduced the axiom of adjunction as one of the axioms for a system of set theory that he introduced in about 1929. It is a weak axiom, used in some weak systems of set theory such as general set theory or finitary set theory. The adjunction operation is also used as one of the operations of primitive recursive set functions.

• 1/2
Views:
718
796
• Adjoining Elements to a Ring Part 2
• Mod-01 Lec-12 Formal Theories

## Interpretability of arithmetic

Tarski and Szmielew showed that Robinson arithmetic (${\displaystyle {\mathsf {Q}}}$) can be interpreted in a weak set theory whose axioms are extensionality, the existence of the empty set, and the axiom of adjunction (Tarski 1953, p.34). In fact, empty set and adjunction alone (without extensionality) suffice to interpret ${\displaystyle {\mathsf {Q}}}$.[1] (They are mutually interpretable.)

Adding epsilon-induction to empty set and adjunction yields a theory that is mutually interpretable with Peano arithmetic (${\displaystyle {\mathsf {PA}}}$). Another axiom schema also yields a theory that is mutually interpretable with ${\displaystyle {\mathsf {PA}}}$:[2]

${\displaystyle \forall x.\forall y.\exists w.\forall z.{\Big (}z\in w\leftrightarrow {\big (}(z\in x\lor z=y)\land \phi {\big )}{\Big )}}$,

where ${\displaystyle \phi }$ is not allowed to have ${\displaystyle w}$ free. This combines axioms of set theory: For ${\displaystyle \phi }$ trivially true it reduced to the adjunction axiom above, and for ${\displaystyle (z\neq y)\land P}$ it gives the axiom of separation with ${\displaystyle P}$.

## References

1. ^ Mancini, Antonella; Montagna, Franco (Spring 1994). "A minimal predicative set theory". Notre Dame Journal of Formal Logic. 35 (2): 186–203. doi:10.1305/ndjfl/1094061860. Retrieved 23 November 2021.
2. ^ Friedman, Harvey M. (June 2, 2002). "Issues in the foundations of mathematics" (PDF). Department of Mathematics. Ohio State University. Retrieved January 18, 2023.
• Bernays, Paul (1937), "A System of Axiomatic Set Theory--Part I", The Journal of Symbolic Logic, 2 (1), Association for Symbolic Logic: 65–77, doi:10.2307/2268862, JSTOR 2268862
• Kirby, Laurence (2009), "Finitary Set Theory", Notre Dame J. Formal Logic, 50 (3): 227–244, doi:10.1215/00294527-2009-009, MR 2572972
• Tarski, Alfred (1953), Undecidable theories, Studies in Logic and the Foundations of Mathematics, Amsterdam: North-Holland Publishing Company, MR 0058532
• Tarski, Alfred; Givant, Steven R. (1987). A Formalization of Set Theory without Variables. AMS Colloquium Publications, v. 41. American Mathematical Soc. ISBN 978-0-8218-1041-5.