Substitution (logic)

Substitution is a fundamental concept in logic.
A substitution is a syntactic transformation on formal expressions.
To apply a substitution to an expression means to consistently replace its variable, or placeholder, symbols by other expressions.
The resulting expression is called a substitution instance, or instance for short, of the original expression.

Propositional logic

Definition

Where ''ψ'' and ''φ'' represent formulas of propositional logic, ''ψ'' is a substitution instance of ''φ'' if and only if ''ψ'' may be obtained from ''φ'' by substituting formulas for symbols in ''φ'', replacing each occurrence of the same symbol by an occurrence of the same formula. For example:

::(R → S) & (T → S)
is a substitution instance of:
::P & Q

and

::(A ↔ A) ↔ (A ↔ A)
is a substitution instance of:
::(A ↔ A)

In some deduction systems for propositional logic, a new expression (a proposition) may be entered on a line of a derivation if it is a substitution instance of a previous line of the derivation (Hunter 1971, p. 118). This is how new lines are introduced in some axiomatic systems. In systems that use rules of transformation, a rule may include the use of a ''substitution instance'' for the purpose of introducing certain variables into a derivation.

In first-order logic, every closed propositional formula that can be obtained from an open propositional formula ''φ'' by substitution is said to be a substitution instance of ''φ''. If ''φ'' is a closed propositional formula we count ''φ'' itself as its only substitution instance.

Tautologies

A propositional formula is a tautology if it is true under every valuation (or interpretation) of its predicate symbols. If Φ is a tautology, and Θ is a substitution instance of Φ, then Θ is again a tautology. This fact implies the soundness of the deduction rule described in the previous section.

First-order logic

In first-order logic, a substitution is a total mapping from variables to terms; many, but not all authors additionally require ''σ''(''x'') = ''x'' for all but finitely many variables ''x''. The notation some authors use ''t''₁/''x''₁, …, ''t''_''k''/''x''_''k'' to denote that substitution, e.g. , here: p. 682;
refers to a substitution mapping each variable ''x''_''i'' to the corresponding term ''t''_''i'', for ''i''=1,…,''k'', and every other variable to itself; the ''x''_''i'' must be pairwise distinct. Applying that substitution to a term ''t'' is written in postfix notation as ''t'' ; it means to (simultaneously) replace every occurrence of each ''x''_''i'' in ''t'' by ''t''_''i''.From a term algebra point of view, the set ''T'' of terms is the free term algebra over the set ''V'' of variables, hence for each substitution mapping σ: ''V'' → ''T'' there is a unique homomorphism : ''T'' → ''T'' that agrees with σ on ''V'' ⊆ ''T''; the above-defined application of ''σ'' to a term ''t'' is then viewed as applying the function to the argument ''t''. The result ''tσ'' of applying a substitution ''σ'' to a term ''t'' is called an instance of that term ''t''.
For example, applying the substitution to the term
:

The domain ''dom''(''σ'') of a substitution ''σ'' is commonly defined as the set of variables actually replaced, i.e. ''dom''(''σ'') = .
A substitution is called a ground substitution if it maps all variables of its domain to ground, i.e. variable-free, terms.
The substitution instance ''tσ'' of a ground substitution is a ground term if all of ''ts variables are in ''σ''s domain, i.e. if ''vars''(''t'') ⊆ ''dom''(''σ'').
A substitution ''σ'' is called a linear substitution if ''tσ'' is a linear term for some (and hence every) linear term ''t'' containing precisely the variables of ''σ''s domain, i.e. with ''vars''(''t'') = ''dom''(''σ'').
A substitution ''σ'' is called a flat substitution if ''xσ'' is a variable for every variable ''x''.
A substitution ''σ'' is called a renaming substitution if it is a permutation on the set of all variables. Like every permutation, a renaming substitution σ always has an inverse substitution ''σ''⁻¹, such that ''tσσ''⁻¹ = ''t'' = ''tσ''⁻¹''σ'' for every term ''t''. However, it is not possible to define an inverse for an arbitrary substitution.

For example, is a ground substitution, is non-ground and non-flat, but linear,
is non-linear and non-flat, is flat, but non-linear, is both linear and flat, but not a renaming, since is maps both ''y'' and ''y''₂ to ''y''₂; each of these substitutions has the set as its domain. An example for a renaming substitution is , it has the inverse . The flat substitution cannot have an inverse, since e.g. (''x''+''y'') = ''z''+''z'', and the latter term cannot be transformed back to ''x''+''y'', as the information about the origin a ''z'' stems from is lost. The ground substitution cannot have an inverse due to a similar loss of origin information e.g. in (''x''+2) = 2+2, even if replacing constants by variables was allowed by some fictitious kind of "generalized substitutions".

Two substitutions are considered equal if they map each variable to structurally equal result terms, formally: ''σ'' = ''τ'' if ''xσ'' = ''xτ'' for each variable ''x'' ∈ ''V''.
The composition of two substitutions ''σ'' = and τ = is obtained by removing from the substitution those pairs ''y''_''i'' ↦ ''u''_''i'' for which ''y''_''i'' ∈ .
The composition of ''σ'' and ''τ'' is denoted by ''στ''. Composition is an associative operation, and is compatible with substitution application, i.e. (''ρσ'')''τ'' = ''ρ''(''στ''), and (''tσ'')''τ'' = ''t''(''στ''), respectively, for every substitutions ''ρ'', ''σ'', ''τ'', and every term ''t''.
The identity substitution, which maps every variable to itself, is the neutral element of substitution composition. A substitution ''σ'' is called idempotent if ''σσ'' = ''σ'', and hence ''tσσ'' = ''tσ'' for every term ''t''. The substitution is idempotent if and only if none of the variables ''x''_''i'' occurs in any ''t''_''i''. Substitution composition is not commutative, that is, στ may be different from τσ, even if σ and τ are idempotent.

For example, is equal to , but different from . The substitution is idempotent, e.g. ((''x''+''y'') ) = ((''y''+''y'')+''y'') = (''y''+''y'')+''y'', while the substitution is non-idempotent, e.g. ((''x''+''y'') ) = ((''x''+''y'')+''y'') = ((''x''+''y'')+''y'')+''y''. An example for non-commuting substitutions is = , but = .

See also

*Substitution property in Equality (mathematics)#Some basic logical properties of equality
* First-order logic#Rules of inference
* Universal instantiation
* Lambda calculus#Substitution
* Truth-value semantics
*Unification (computer science)
* Metavariable
* Mutatis mutandis
*Rule of replacement
* Substitution (algebra) — about applying substitutions to polynoms and other algebraic expressions
*String interpolation — as seen in computer programming

Notes

References

* Crabbé, M. (2004).
On the Notion of Substitution
'. Logic Journal of the IGPL, 12, 111–124.
* Curry, H. B. (1952)
On the definition of substitution, replacement and allied notions in an abstract formal system
'. Revue philosophique de Louvain 50, 251–269.
* Hunter, G. (1971). ''Metalogic: An Introduction to the Metatheory of Standard First Order Logic''. University of California Press.
* Kleene, S. C. (1967). ''Mathematical Logic''. Reprinted 2002, Dover.

External links

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{{DEFAULTSORT:Substitution (Logic)

Propositional calculus
Concepts in logic
Logical truth
Automated theorem proving
Logic programming