tailieunhanh - Handbook of Industrial Automation - Richard L. Shell and Ernest L. Hall Part 2

Tham khảo tài liệu 'handbook of industrial automation - richard l. shell and ernest l. hall part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Equivalence Relations Now we concentrate our attention on the properties of a binary relation defined in a set X. 1. is called reflexive in X if and only if for all X e X xTx. 2. is called symmetrical in X if and only if for all X y e X x implies j Tx. 3. is called transitive in X if and only if for all X y z e X x and yffiz implies x z. A binary relation is called an equivalence relation on X if it is reflexive symmetrical and transitive. As an example consider the set z of integer numbers and let n be an arbitrary positive integer. The congruence relation modulo n on the set z is defined by X y modulo n if and only if X y kn for some k e z. The congruence relation is an equivalence relation on z. Proof 1. For each X e z X X 0 . This means that X X modulo n which implies that the congruence relation is reflexive. 2. If X y modulo n X y kn for some k e z. Multiplying both sides of the last equality by 1 we get y X kn which implies that y X modulo n . Thus the congruence relation is symmetrical. 3. If X y modulo n and y z modulo n we have X y kỵ n and y z k2n for some kỵ and k2 in z. Writing X z X y y z we get X z kỵ k2 n. Since kỵ k2 e z we conclude that X z modulo n . This shows that the congruence relation is transitive. From 1-3 it follows that the congruence relation modulo n is an equivalence relation on the set z of integer numbers. In particular we observe that if we choose n 2 then X y modulo 2 means that X y 2k for some integer k. This is equivalent to saying that either X and y are both even or both X and y are odd. In other words any two even integers are equivalent any two odd integers are equivalent but an even integer can not be equivalent to an odd one. The set z has been divided into two disjoint subsets whose union gives z. One such proper subset is the set of even integers and the other one is the set of odd integers. Partitions and Equivalence Relations The situation described in the last example is quite general. To study .

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