# Definition:Sign (Mathematical Theory)

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## Definition

A **sign** of a mathematical theory is a symbol which has a specific meaning in the theory, as follows:

- A logical sign;
- A letter;
- A specific sign.

A logical sign is a sign which has a specific meaning and definition in a particular class of mathematical theory.

A letter is a more or less arbitrary symbol whose definition depends on the specific context.

For a given mathematical theory, there are signs which are specific to the use to which the theory is being put. These are the specific signs.

## Sources

- 1968: Nicolas Bourbaki:
*Theory of Sets*... (next): Chapter $\text I$: Description of Formal Mathematics: $1$. Terms and Relations: $1$. Signs and Assemblies