Using PL to Describe Properties of Systems
Generic description of a propositional calculus
A propositional calculus is a formal system , where:
- The alpha set is a countably infinite set of elements called proposition symbols or propositional variables. Syntactically speaking, these are the most basic elements of the formal language , otherwise referred to as atomic formulas or terminal elements. In the examples to follow, the elements of are typically the letters p, q, r, and so on.
- The omega set Ω is a finite set of elements called operator symbols or logical connectives. The set Ω is partitioned into disjoint subsets as follows:
In this partition, is the set of operator symbols of arity j.
In the more familiar propositional calculi, Ω is typically partitioned as follows:
A frequently adopted convention treats the constant logical values as operators of arity zero, thus:
Some writers use the tilde (~), or N, instead of ¬; and some use v instead of as well as the ampersand (&), the prefixed K, or instead of . Notation varies even more for the set of logical values, with symbols like {false, true}, {F, T}, or {0, 1} all being seen in various contexts instead of .
- The zeta set is a finite set of transformation rules that are called inference rules when they acquire logical applications.
- The iota set is a countable set of initial points that are called axioms when they receive logical interpretations.
The language of , also known as its set of formulas, well-formed formulas, is inductively defined by the following rules:
- Base: Any element of the alpha set is a formula of .
- If are formulas and is in , then is a formula.
- Closed: Nothing else is a formula of .
Repeated applications of these rules permits the construction of complex formulas. For example: