Equational logic

First-order equational logic consists of quantifier-free terms of ordinary first-order logic, with equality as the only predicate symbol. The model theory of this logic was developed into universal algebra by Birkhoff, Grätzer, and Cohn. It was later made into a branch of category theory by Lawvere ("algebraic theories").

The terms of equational logic are built up from variables and constants using function symbols (or operations).

Syllogism

Here are the four inference rules of logic. ${\textstyle P[x:=E]}$ denotes textual substitution of expression ${\textstyle E}$ for variable ${\textstyle x}$ in expression ${\textstyle P}$. Next, ${\textstyle b=c}$ denotes equality, for ${\textstyle b}$ and ${\textstyle c}$ of the same type, while ${\textstyle b\equiv c}$, or equivalence, is defined only for ${\textstyle b}$ and ${\textstyle c}$ of type boolean. For ${\textstyle b}$ and ${\textstyle c}$ of type boolean, ${\textstyle b=c}$ and ${\textstyle b\equiv c}$ have the same meaning.

Substitution	If ${\textstyle P}$ is a theorem, then so is ${\textstyle P[x:=E]}$.	$$\vdash P\qquad \rightarrow \qquad \vdash P[x:=E]$$
Leibniz	If ${\textstyle P=Q}$ is a theorem, then so is ${\textstyle E[x:=P]=E[x:=Q]}$.	$$\vdash P=Q\qquad \rightarrow \qquad \vdash E[x:=P]=E[x:=Q]$$
Transitivity	If ${\textstyle P=Q}$ and ${\textstyle Q=R}$ are theorems, then so is ${\textstyle P=R}$.	$$\vdash P=Q,\;\vdash Q=R\qquad \rightarrow \qquad \vdash P=R$$
Equanimity	If ${\textstyle P}$ and ${\textstyle P\equiv Q}$ are theorems, then so is ${\textstyle Q}$.	$$\vdash P,\;\vdash P\equiv Q\qquad \rightarrow \qquad \vdash Q$$

Proof

We explain how the four inference rules are used in proofs, using the proof of ${\textstyle \lnot p\equiv p\equiv \bot }$. The logic symbols ${\textstyle \top }$ and ${\textstyle \bot }$ indicate "true" and "false," respectively, and ${\textstyle \lnot }$ indicates "not." The theorem numbers refer to theorems of A Logical Approach to Discrete Math.

$${\begin{array}{lcl}(0)&&\lnot p\equiv p\equiv \bot \\(1)&=&\quad \left\langle \;(3.9),\;\lnot (p\equiv q)\equiv \lnot p\equiv q,\;{\text{with}}\ q:=p\;\right\rangle \\(2)&&\lnot (p\equiv p)\equiv \bot \\(3)&=&\quad \left\langle \;{\text{Identity of}}\ \equiv (3.9),\;{\text{with}}\ q:=p\;\right\rangle \\(4)&&\lnot \top \equiv \bot &(3.8)\end{array}}$$

First, lines ${\textstyle (0)}$–${\textstyle (2)}$ show a use of inference rule Leibniz:

$$(0)=(2)$$

is the conclusion of Leibniz, and its premise ${\textstyle \lnot (p\equiv p)\equiv \lnot p\equiv p}$ is given on line ${\textstyle (1)}$. In the same way, the equality on lines ${\textstyle (2)}$–${\textstyle (4)}$ are substantiated using Leibniz.

The "hint" on line ${\textstyle (1)}$ is supposed to give a premise of Leibniz, showing what substitution of equals for equals is being used. This premise is theorem ${\textstyle (3.9)}$ with the substitution ${\textstyle p:=q}$, i.e.

$$(\lnot (p\equiv q)\equiv \lnot p\equiv q)[p:=q]$$

This shows how inference rule Substitution is used within hints.

From ${\textstyle (0)=(2)}$ and ${\textstyle (2)=(4)}$, we conclude by inference rule Transitivity that ${\textstyle (0)=(4)}$. This shows how Transitivity is used.

Finally, note that line ${\textstyle (4)}$, ${\textstyle \lnot \top \equiv \bot }$, is a theorem, as indicated by the hint to its right. Hence, by inference rule Equanimity, we conclude that line ${\textstyle (0)}$ is also a theorem. And ${\textstyle (0)}$ is what we wanted to prove.

See also

• Theory of pure equality

External links

• Sakharov, Alex. "Equational Logic." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein.