formal system
In mathematical logic, a formal system is a “set-up” to study the syntactic structures of statements that we see in everyday mathematics. Thus, instead of looking at the meaning of the statement , one looks at the expression made up of symbols , and .
Formally, a formal system consists of the following:
- 1.
the language
of , which is formed by
- (a)
a set of symbols; finite sequences
of these symbols are called words or expressions over ;
- (b)
a set of expressions called well-formed formulas, or simply formulas
;
- (c)
a set of rules that specify how these words are generated; the set is called the syntax of the language; typically, the rules are
- *
certain expressions are singled out, which are called atoms, or atomic formulas
- *
a set of rules (inductive in nature) showing how formulas can be formed from other formulas that have already been formed
- *
- (a)
- 2.
a deductive system (or proof system) associated with , which consists of
- (a)
a set of axioms; each axiom is usually (but not always) a formula;
- (b)
a set of rules called rules of inference
on ; the rules of inferences are used to generate formulas, which are known as theorems of ; typically,
- *
an axiom is a theorem,
- *
a formula that can be formed (or deduced) from other theorems by a rule of inference is a theorem.
- *
- (a)
Thus, in a formal system , one starts with a set of symbol. Three sets are associated:
where is the set of all expressions over , is the set of all (well-formed) formulas obtained by a set of formula formation rules, and is the set of all theorems obtained from the formulas by a set of rules of inference.
Here’s an example of a formal system: the (classical) propositional logic . The symbols of consists of
(not including the symbol for commas, and a set of elements called propositional variables. Formulas of are generated as follows:
- •
propositional variables are formulas; and
- •
if are formulas, so are
A deduction system for may consist the following:
- •
all formulas of the forms
for all formulas , and
- •
, and the (only) rule of inference is modus ponens
: from and , we may infer (or deduce) .
The (classical) first-order logic is another example of a formal system. Group theory, number theory (a la Peano arithmetic), theory of partially ordered sets
, and set theory
(a la ZFC) are other examples of formal systems that are extensively studied.
Remark. A formal system is sometimes called a logical system, although the latter term usually either refers to a formal system where the deducibility relation arising out of the associated deductive system satisfies certain properties (for example, for all wffs, etc…), or a formal system together with a semantical structure.
References
- 1 R. M. Smullyan,Theory of Formal Systems, Princeton University Press, 1961
- 2 J. R. Shoenfield,Mathematical Logic, AK Peters, 2001