In universal algebra, a quasi-identity is an implication of the form
- s1 = t1 ∧ … ∧ sn = tn → s = t
where s1, ..., sn, s and t1, ..., tn,t are terms built up from variables using the operation symbols of the specified signature.
Quasi-identities amount to conditional equations for which the conditions themselves are equations. A quasi-identity for which n = 0 is an ordinary identity or equation, whence quasi-identities are a generalization of identities. Quasi-identities are special type of Horn clauses.
- Burris, Stanley N.; H.P. Sankappanavar (1981). A Course in Universal Algebra. Springer. ISBN 3-540-90578-2. Free online edition.
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