2.1 Sets 2.2 Set Operations Set Operations Venn Diagrams Set Identities

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# 2.1 Sets 2.2 Set Operations Set Operations Venn Diagrams Set Identities

## 2.1 Sets 2.2 Set Operations Set Operations Venn Diagrams Set Identities

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1. 2.1 Sets • 2.2 Set Operations • Set Operations • Venn Diagrams • Set Identities • Union and Intersection of Indexed Collections • 2.3 Functions • 2.4 Sequences and Summations P. 1

2. Propositional calculus and set theory are both instances ofan algebraic system called aBoolean Algebra. The operators in set theory are defined in terms of thecorresponding operator in propositional calculus. As always there must be a universe U. All sets areassumed to be subsets of U. Set Operations P. 1

3. Definition: Two sets A and B are equal, denoted A = B,iffx[x A↔x B]. Note: By a previous logical equivalence we have A = B iffx[(x A→ x B) Λ (x B → x A)] or A = BiffA B and B A Set Operations P. 1

4. Definitions: The union of A and B, denoted AB, is the set {x | xAV xB} The intersection of A and B, denoted AB, is the set {x | xAΛxB} Set Operations FIGURE 1 FIGURE 2 P. 1

5. Note: If the intersection is void, A and B are said to be disjoint. The complement of A, denoted , is the set {x | x  A}={x | ¬(xA)} Note: Alternative notation is , and {x|xA}. The difference of A and B, or the complement of B relative to A, denoted A - B, is the set A  Note: The complement of A is U - A. The symmetric difference of A and B, denoted AB, is the set (A - B)(B - A). Set Operations P. 1

6. Examples: U = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10} A= {1, 2, 3, 4, 5}, B = {4, 5, 6, 7, 8}. Then AB = {1, 2, 3, 4, 5, 6, 7, 8} AB = {4, 5} = {0, 6, 7, 8, 9, 10} = {0, 1, 2, 3, 9, 10} A - B = {1, 2, 3} B - A = {6, 7, 8} A B = {1, 2, 3, 6, 7, 8} Set Operations P. 1

7. A useful geometric visualization tool (for 3 or less sets) The Universe U is the rectangular box. Each set is represented by a circle and its interior. All possible combinations of the sets must be represented. Shade the appropriate region to represent the given set operation. Venn Diagrams P. 1

8. Set identities correspond to the logical equivalences. Example: The complement of the union is the intersection of the complements Set Identities P. 1

9. Set Identities • Proof: To show To show two sets are equal we show for all x that x is a member of one set if and only if it is a member of the other. • We now apply an important rule of inference (defined later) called Universal Instantiation

10. Set Identities • Universal Instantiation In a proof we can eliminate the universal quantifier which binds a variable if we do not assume anything about the variable other than it is an arbitrary member of the Universe. We can then treat the resulting predicate as a proposition.

11. We say 'Let x be arbitrary. ' Then we can treat the predicates as propositions: Assertion Reason Def. of complement ¬ Def. of   ¬ Def. of union ¬xAΛ ¬xBDeMorgan's Laws Def. of  Def. of complement Def. of intersection Hence is a tautology. Set Identities P. 1

12. Since x was arbitrary we have used only logically equivalent assertions and definitions we can apply another rule of inference called Universal Generalization We can apply a universal quantifier to bind a variable if we have shown the predicate to be true for all values of the variable in the Universe. and claim the assertion is true for all x, i.e., Set Identities P. 1

13. Set Identities • Q. E. D. (an abbreviation for the Latin phrase “Quod Erat Demonstrandum” - “which was to be demonstrated” used to signal the end of a proof) • Note: As an alternative which might be easier in some cases, use the identity

14. Example: Show A(B - A) = Ø The void set is a subset of every set.Hence, A (B - A)  Ø Therefore, it suffices to show A(B - A) Ø or x[x A (B - A) → x Ø] Set Identities P. 1

15. Set Identities So as before we say 'let x be arbitrary’. Show x  A  (B- A) → x  Ø is a tautology. But the consequent is always false. Therefore, the antecedent better always be false also.

16. Solution: Apply the definitions: Assertion Reason x A(B- A)x  A Λ x  (B- A) by Def. of   x  A Λ (x  B Λ x A) by Def. of - (x A Λx  A) Λx B by Def. of Associative  0 Λx B by Def. of Complement  0 (false) by Def. of Domination Set Identities P. 1

17. Let A1,A2 ,..., Anbe an indexed collection of sets. Definition: The union of a collection of sets is the set that contains those elements that are members of at least one set in the collection. we use the notation to denote the union of the sets A1,A2 ,..., An Union and Intersection of Indexed Collections P. 1

18. Union and Intersection of Indexed Collections • Definition: The intersection of a collection of sets is the set that contains those elements that are members of all the set in the collection. we use the notation to denote the intersection of the sets A1,A2 ,..., An

19. Examples: Let Union and Intersection of Indexed Collections P. 1

20. Boolean Algebra Set operations Union Intersection Complement Difference Terms • Symmetric difference • Venn Diagram • Set Identities • Universal Instantiation • Universal Generalization P. 1