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A sign of a formal language $\LL$ is a symbol whose primary purpose is to structure the language.
In building a formal language, signs form the hooks allowing the formal grammar to define the well-formed formulae of the formal language.
Common examples of signs are parentheses, "(" and ")", and the comma, ",".
The logical connectives are also signs.
Signs form part of the alphabet of a formal language.
Unlike the letters, they must be the same for each signature for the language.
Compare with letters, which also form part of the alphabet.
- 1968: Nicolas Bourbaki: Theory of Sets ... (next): Chapter $\text I$: Description of Formal Mathematics: $1$. Terms and Relations: $1$. Signs and Assemblies