Models in Science - Northwestern University

Any structure of which all these statements are true is a model of Euclidean geometry. A structure S = is a composite entity consisting of (i) a non-empty set U of individuals called the domain (or universe) of S, (ii) an indexed set O (i.e. an ordered list) of operations on U (which may be empty), and (iii) a non-empty indexed set R ... ................
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