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Signature in First-Order Logic

In first-order logic, a signature is a set of symbols of two kinds of constants:

  1. Function constants
  2. Predicate constants

Each of these has a nonnegative integer, called the arity, assigned to each symbol.

Moreover,

  • Function constants of arity 0 are called object constants
  • Predicate constants of arity 0 are called propositional constants.