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单词 algebra of sets
释义

algebra of sets

The power set ℘(E) of all subsets of a universal set E is closed under the binary operations ∪(union) and ∩(intersection), + (symmetric difference) and the unary operation ′ (complementation). The following are some of the properties, or laws, that hold for subsets A, B, and C of E:

(i) A∪(B ∪ C)=(A ∪ B)∪C, A + (B + C)=(A + B) + C and A∩(B ∩ C)=(A ∩ B)∩C, the associative properties.
(ii) A ∪ B = B ∪ A, A + B = B + A, and A ∩ B = B ∩ A, the commutative properties.
(iii) A∪Ø = A, A + Ø = A and A∩Ø= Ø, where Ø is the empty set.
(iv) A ∪ E = E and A ∩ E = A.
(v) A ∪ A = A, A + A = Ø and A ∩ A = A.
(vi) A∩(B ∪ C)=(A ∩ B)∪(A∩∩C), A + (B ∩ C)=(A + B)∩(A + C), and A∪(B ∩ C)=(A ∪ B)∩(A ∪ C), the distributive properties.
(vii) A ∪ A′ = E, A + A′ = E, and A ∩ A′=Ø.
(viii) E′=Ø and Ø′ = E.
(ix) (A′)′ = A.
(x) (A ∪ B)′ = A′∩B′ and (A ∩ B)′ = A′∪B′, De Morgan's laws.

The application of these laws to subsets of E is known as the algebra of sets. Despite some similarities with the algebra of numbers, there are important and striking differences. If |E| = n then (℘(E), +, ∩) is isomorphic as a ring to (ℤ2n, +, ×). See Boolean algebra.

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更新时间:2025/4/30 3:42:26