Generically, an alternative set theory is an alternative mathematical approach to the concept of set. It is a proposed alternative to the standard set theory.

Some of the alternative set theories are:

• the theory of semisets;
• the set theory New Foundations
• Positive set theory
• Internal set theory

Specifically, Alternative Set Theory (or AST) refers to a particular set theory developed in the 1970s and 1980s by Petr Vop?nka and his students. It builds on some ideas of the theory of semisets, but also introduces more radical changes: for example, all sets are "formally" finite, which means that sets in AST satisfy the law of mathematical induction for set-formulas (more precisely: the part of AST that consists of axioms related to sets only is equivalent to the Zermelo?Fraenkel (or ZF) set theory, in which the axiom of infinity is replaced by its negation). However, some of these sets contain subclasses that are not sets, which makes them different from Cantor (ZF) finite sets and they are called infinite in AST.

See also

• Non-well-founded set theory

References

• Vop?nka, P. Mathematics in the Alternative Set Theory. Teubner, Leipzig, 1979.
• Proceedings of the 1st Symposium Mathematics in the Alternative Set Theory. JSMF, Bratislava, 1989.
• Holmes, Randall M. Alternative Set Theories., 2006.

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