Comparing Sets MATH 102 Contemporary Math S. Rook
Overview • Section 2.2 in the textbook: • Set equality & set equivalence • Subsets
Set Equality & Set Equivalence
Set Equality • Two sets, A and B, are equal, denoted as A = B if they both contain exactly the same elements; otherwise, we write A ≠ B • Order DOES NOT matter • e.g. Let A = {1, 2, 3, 4, 5} and B = {5, 4, 3, 2, 1}. Does A = B? • If A = B, what can we say about n(A) and n(B)?
Set Equivalence • Two sets, A and B, are equivalent if n(A) = n(B) • i.e. the number of elements in each is the same • Set equality is NOT the same as set equivalence!!! • You must understand the difference! • e.g. Consider any finite set A • List the elements in set B so that AequalsB • List the elements in set B so that A is equivalent, but NOTequal to B
Set Equality (Example) Ex 1: Replace # with = or ≠ to make the statement true: a) {2, 3, 5, 7} # {x | x is a prime number less than 12} b) {y | y is a weekday} # {Friday, Monday, Thursday, Tuesday, Wednesday}
Subsets
Subsets • We say that A is a subset of B, denoted by if EVERY element of A is also in B • Again, order does NOT matter • e.g. Let A = {2, 6, 8, 10} and B = {14, 12, 10, 8, 6, 2}. Is A a subset of B? • If there is at least one element of A that is not in B, we write A n/s B • e.g. Consider sets A and B from above. Is B a subset of A?
Subsets (Continued) • Given sets A and B, if A is a subset of BANDA ≠ B, we say that A is a proper subset of B denoted • Note that BOTH conditions must be fulfilled for A to be a proper subset of B • e.g. Let A = {a, e, i, o, u} and B = { l | l is a letter of the alphabet }. Is A a proper subset of B? • e.g. Let A = {a, e, i, o, u} and B = {v | v is a vowel}. Is A a proper subset of B?
Subsets (Example) Ex 2: Replace the # with to make the statement true: a) {t | t is a letter in the word ruth} # {z | z is a letter in the word truth} b) Ø # {1, 2, 3, …, 100} c) {Aberdeen, Darlington, Fallston} # {b | b is a building at HCC}
Listing Subsets • Sometimes it is useful to know all subsets of a set in order to assist in making decisions • See the options discussion on pg 49-50 of the textbook • For any set A: • The leastnumber of elements in A’s subsets is 0 • How do we write a set with 0 elements? • The maximum number of elements in A’s subsets is n(A) • To list the subsets of A, we first list Ø and A and then list the subsets that have between 0 and n(A) elements • When n(B) = n(C), for any two subsets B and C of A, B ≠ C • i.e. Same-sized subsets must have different elements • e.g. Consider listing the subsets of A = {a, b}
Listing Subsets (Continued) • Now consider listing the subsets of B = {a, b, c} • What is the relationship between the number of subsets of a set with 2 elements versus a set with 3 elements? • The number of subsets of a set containing k elements is 2k • Consider again our subset listings for sets A and B • How many proper subsets are in each listing? • What is the relationship between the number of proper subsets of a set with 2 elements versus a set with 3 elements? • The number of proper subsets of a set containing k elements is 2k – 1
Listing Subsets (Example) Ex 3: The board of directors of a corporation own different amounts of stock which affects voting power. Adam has a voting power of 4, Beth has a voting power of 3, Chris 2, and Danielle 1. Any issue needs a voting weight of at least 6 to be passed. List all of the different possible voting combinations where an issue passes.
Summary • After studying these slides, you should know how to do the following: • Given two sets A and B, determine whether they are equal or equivalent • Given two sets A and B, determine whether A is a subset, is not a subset, or is a proper subset of B • List all of the subsets of a given set A • Additional Practice: • See the list of suggested problems for 2.2 • Next Lesson: • Set Operations (Section 2.3)
