# Tag Info

Questions about mathematical logic, including model theory, proof theory, computability theory (a.k.a. recursion theory), and non-standard logics. Questions which merely seek to apply logical or formal reasoning to other areas of mathematics should not use this tag. Consider using one of the following tags as well, if they fit the question: (model-theory), (set-theory), (computability), and (proof-theory). This tag is not for logical puzzles, use (puzzle).

The field of logic in mathematics deals with questions regarding the nature of formalized mathematical statements, proofs of statements, the structure of objects that are described by these statements, as well as questions regarding computability.

More precisely, mathematical logic studies questions that ask:

• How can we represent mathematical statements formally? How expressive must the language used be? Can certain properties, theorems, etc. be formulated in more restricted settings than others?
• What, exactly, is a proof? What do valid conclusions in proofs look like? For a given language, can we find an effective/algorithmic proof generation method?
• What can be said about the objects that are described by a set of formal statements? For example, can we assume that the objects will be unique? Will they have a fixed cardinality? What will be the common properties of all such objects?

A related subject is computability: Given a mathematically precise problem description, is there an algorithm that solves this problem? If not, why not?

What this tag does not include are basic questions about informal uses of logic - using mathematical arguments to prove some statement should be labeled with the tag instead.

A more detailed description can be found at MathWorld or The Encyclopedia of Mathematics.

This tag should not be used for what a layperson might called "a logical puzzle". For these sort of questions please use and as appropriate. (Unless the solution is done via a method relevant to the logic tag, of course.)