Sets and Subsets PDF

1.1 Sets and Subsets 1  Definitions of sets A set is any well-defined collection of objects The elements or members of a set are the objects contained in the set Well-defined means that it is possible to decide if a given object belongs to the collectio...

1.1 Sets and Subsets 1  Definitions of sets A set is any well-defined collection of objects The elements or members of a set are the objects contained in the set Well-defined means that it is possible to decide if a given object belongs to the collection or not. 2  Set notation Enumeration of sets are represented with a list of elements in curly brackets – example: {1, 2, 3} Set labels are uppercase letters in italics – example: A, B, C Element labels are lowercase letters – example: a, b, c  is the label for the empty set, i.e.,  = { } 3  Membership  -- “is an element of” (Note that it is shaped like an “E” as in element)  -- “is not an element of” Example: If A = {1, 3, 5, 7}, then 1  A, but 2  A. 4  Specifying sets with their properties: A set can be represented or defined by the rules classifying whether an object belongs to a collection or not. A = {a | a is __________} The above notation translates to “the set A is comprised of elements a where a satisfies _________.” 5  Specifying sets with their properties: * Example: A = {x | x is a positive integer less than 4}, Then A = {1, 2, 3} * Example: B = {x | x is the letter in the word “byte”}, Then B = {b, y, t, e} 6  Examples of Common Sets Z+ = {x | x is a positive integer} N = {x | x is a positive integer or zero} Z = {x | x is an integer} Q = {x | x is a rational number} Q consists of the numbers that can be written a/b, where a and b are integers and b ≠ 0. R = {x | x is a real number} 7 8  Subsets If every element of A is also an element of B, that is, if whenever x  A, then x  B, we say that A is a subset of B or that A is contained in B. A is a subset of B if for every x, x  A means that x  B. 9  Subset Notation  -- “is contained in” (Note that it is shaped like a “C” as in contained in)  -- “is not contained in” “Is not contained in” does not mean that there aren’t some elements that can be in both sets. It just means that not all of the elements of A are in B 10  Subset Examples vowels  alphabet letters that spell “see”  letters that spell “yes” letters that spell “yes”  letters that spell “easy” letters that spell “say”  letters that spell “easy” positive integers  integers odd integers  integers integers  floating point values (real numbers) 11  Venn Diagrams Named after British logician John Venn Graphical depiction of the relationship of sets. Does not represent the individual elements of the sets, rather it implies their existence 12  Venn Diagram Examples AB B A AB A B B or A 13 More Venn Diagram Examples a  A, b  B, and c  A and c  B A B c a b 14  Theorems on Sets A  B and B  C implies A  C Example: – A = {x | letters that spell “see”} = {e, s} – B = {x | letters that spell “yes”} = {e, s, y} – C = {x | letters that spell “easy”} = {a, e, s, y} 15  Theorems on Sets (continued) If A  B and B  C, then A  C. If A  B and C  B, that doesn’t mean we can say anything at all about the relationship between A and C. It could be any of the following three cases: B A C B A C B A C 16  Theorems on Sets (continued) If A is any set, then A  A. That is, every set is a subset of itself. Since  contains no elements, then every element of  is contained in every set. Therefore, if A is any set, the statement   A is always true. If A  B and B  A, then A = B 17  Final set of terms Finite – A set A is called finite if it has n distinct elements, where n  N. n is called the cardinality of A and is denoted by |A|, e.g., n = |A| Infinite – A set that is not finite is called infinite. The power set of A is the set of all subsets of A including  and is denoted P(A) 18 Final set of terms 19