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Set Theory

Set Theory. Notation. S={a, b, c} refers to the set whose elements are a, b and c . a S means “a is an element of set S” . d S means “d is not an element of set S” . {x S | P(x) } is the set of all those x from S such that P(x) is true. E.g., T={x  Z | 0<x<10 } .

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Set Theory

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  1. Set Theory

  2. Notation • S={a, b, c} refers to the set whose elements are a, b and c. • aS means “a is an element of set S”. • dS means “d is not an element of set S”. • {x S | P(x)} is the set of all those x from S such that P(x) is true. E.g., T={x Z | 0<x<10} . • Notes: 1) {a,b,c}, {b,a,c}, {c,b,a,b,b,c} all represent the same set. 2) Sets can themselves be elements of other sets, e.g., S={ {Mary, John}, {Tim, Ann}, …}

  3. B A Relations between sets • Definition: Suppose A and B are sets. Then A is called a subset of B: A  B iff every element of A is also an element of B. Symbolically, A B  x, if xA then x B. • A  B  x such that xA and xB. A A B B A  B A  B A  B

  4. Relations between sets • Definition: Suppose A and B are sets. Then A equals B: A = B iff every element of A is in B and every element of B is in A. Symbolically, A=B  AB and BA . • Example: Let A = {mZ | m=2k+3 for some integer k}; B = the set of all odd integers. Then A=B.

  5. Operations on Sets Definition: Let A and B be subsets of a set U. 1. Union of A and B: A  B = {xU | xA or xB} 2. Intersection of A and B: A  B = {xU | xA and xB} 3. Difference of B minus A: BA = {xU | xB and xA} 4. Complement of A: Ac = {xU | xA} Ex.: Let U=R, A={x R | 3<x<5}, B ={x R| 4<x<9}. Then 1) A  B = {x R | 3<x<9}. 2) A  B = {x R | 4<x<5}. 3) BA = {x R | 5 ≤x<9}, AB = {x R | 3<x ≤4}. 4) Ac = {xR | x ≤3 or x≥5}, Bc = {xR | x ≤4 or x≥9}

  6. Properties of Sets • Theorem 1 (Some subset relations): 1) AB  A 2) A  AB 3) If A  B and B  C, then A  C . • To prove that A  B use the “element argument”: 1. suppose that x is a particular but arbitrarily chosen element of A, 2. show that x is an element of B.

  7. Proving a Set Property • Theorem 2 (Distributive Law): For any sets A,B and C: A  (B C) = (A  B)  (A  C) . • Proof: We need to show that (I)A  (B  C)  (A  B)  (A  C) and (II) (A  B)  (A  C)  A  (B  C) . Let’s show (I). Suppose x  A  (B  C) (1) We want to show that x  (A  B)  (A  C) (2)

  8. Proving a Set Property • Proof (cont.): x  A  (B  C)  x  A or x  B  C . (a) Let x  A. Then x  AB and x  AC  x  (A  B)  (A  C) (b) Let x  B  C. Then xB and xC. Thus, (2) is true, and we have shown (I). (II) is shown similarly (left as exercise). ■

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