1. Lecture 2.2: Set Theory* CS 250, Discrete Structures, Fall 2011 Nitesh Saxena *Adopted from previous lectures by Cinda Heeren

2. 9/1/2011Lecture 2.2 -- Set Theory2 Course Admin Slides from previous lectures all posted HW1 Posted Due at 11am 09/09/11 Please follow all instructions Recall: late submissions will not be accepted Word Equation editor; Open Office; Alt-Codes Please pick up your competency exams, if you haven’t done so

3. 9/1/2011Lecture 2.2 -- Set Theory3 Outline Set Theory, Operations and Laws

4. 9/1/2011Lecture 2.2 -- Set Theory4 Set Theory - Operators The symmetric difference, A  B, is: A  B = { x : (x  A  x  B) v (x  B  x  A)} = (A - B) U (B - A) like “exclusive or” A U B

5. 9/1/2011Lecture 2.2 -- Set Theory5 Set Theory - Operators A  B = { x : (x  A  x  B) v (x  B  x  A)} = (A - B) U (B - A) Proof:{ x : (x  A  x  B) v (x  B  x  A)} = { x : (x  A - B) v (x  B - A)} = { x : x  ((A - B) U (B - A))} = (A - B) U (B - A)

6. 9/1/2011Lecture 2.2 -- Set Theory6 Set Theory - Famous Laws Two pages of (almost) obvious. One page of HS algebra. One page of new. Don’t memorize them, understand them! They’re in Rosen, p. 130

7. 9/1/2011Lecture 2.2 -- Set Theory7 Set Theory - Famous Laws Identity Domination Idempotent A  U = A A U  = A A U U = U A   =  A U A = A A  A = A

8. 9/1/2011Lecture 2.2 -- Set Theory8 Set Theory - Famous Laws Excluded Middle Uniqueness Double complement A U A = U A  A =  A = A

9. 9/1/2011Lecture 2.2 -- Set Theory9 Set Theory – Famous Laws Commutativity Associativity Distributivity A U B = (A U B) U C = A  B = B U A B  A (A  B)  C = A U (B U C) A  (B  C) A U (B  C) = A  (B U C) = (A U B)  (A U C) (A  B) U (A  C)

10. 9/1/2011Lecture 2.2 -- Set Theory10 Set Theory – Famous Laws DeMorgan’s I DeMorgan’s II pq Venn Diagrams are good for intuition, but we aim for a more formal proof. (A U B) = A  B (A  B) = A U B

11. 9/1/2011Lecture 2.2 -- Set Theory11 3 Ways to prove Laws or set equalities Show that A  B and that A  B. Use a membership table. Use logical equivalences to prove equivalent set definitions. New & importantLike truth tablesNot hard, a little tedious

12. 9/1/2011Lecture 2.2 -- Set Theory12 Example – the first way Prove that 1. (  ) (x  A U B)  (x  A U B)  (x  A and x  B)  (x  A  B) 2. (  ) (x  A  B)  (x  A and x  B)  (x  A U B)  (x  A U B) (A U B) = A  B

13. 9/1/2011Lecture 2.2 -- Set Theory13 Example – the second way Prove that using a membership table. 0 : x is not in the specified set 1 : otherwise (A U B) = A  B ABABA  BAUB 1100010 1001010 0110010 0011101

14. 9/1/2011Lecture 2.2 -- Set Theory14 Example – the third way Prove that using logically equivalent set definitions. (A U B) = A  B (A U B) = {x :  (x  A v x  B)} = {x :  (x  A)   (x  B)} = A  B = {x : (x  A)  (x  B)}

15. 9/1/2011Lecture 2.2 -- Set Theory15 Another example: applying the laws X  (Y - Z) = (X  Y) - (X  Z). True or False? Prove your response. = (X  Y)  (X’ U Z’) = (X  Y  X’) U (X  Y  Z’) =  U (X  Y  Z’) = (X  Y  Z’) (X  Y) - (X  Z) = (X  Y)  (X  Z)’

16. 9/1/2011Lecture 2.2 -- Set Theory16 Suppose to the contrary, that A  B  , and that x  A  B. A Proof (direct and indirect) Pv that if (A - B) U (B - A) = (A U B) then Then x cannot be in A-B and x cannot be in B-A. But x is in A U B since (A  B)  (A U B). A  B =  Thus, A  B = . a)A U B =  b)A = B c)A  B =  d)A-B = B-A =  Then x is not in (A - B) U (B - A).

17. 9/1/2011Lecture 2.2 -- Set Theory17 Today’s Reading Rosen 2.1 and 2.2