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2.1 Sets ‒Sets ‒Common Universal Sets ‒Subsets 2.2 Set Operations 2.3 Functions 2.4 Sequences and Summations 1

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A set is a collection or group of objects or elements or members. (Cantor 1895) – A set is said to contain its elements. – There must be an underlying universal set U, either specifically stated or understood. Sets 2

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Notation: – list the elements between braces: S = {a, b, c, d}={b, c, a, d, d} (Note: listing an object more than once does not change the set. Ordering means nothing.) – specification by predicates: S = {x| P(x)} S contains all the elements from U which make the predicate P true. – brace notation with ellipses: S = {..., -3, -2, -1}, the negative integers. 3

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R = reals N = natural numbers = {0,1, 2, 3,... }, the counting numbers. Z = all integers = {.., -3, -2, -1, 0, 1, 2, 3,4,..}. Z + ={1,2,3,…},the set of positive integers. Q={P/q | p Z,q Z and q≠0},the set of rational numbers. Q + ={x R | x=p/q, for some positive integers p and q} Common Universal Sets 4

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Notation: x is a member of S or x is an element of S: x S x is not an element of S: x S Common Universal Sets 5

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Definition: The set A is a subset of the set B, denoted A B, iff x[x A→x B] Definition: The void set, the null set, the empty set, denoted Ø, is the set with no members. Note: the assertion x Ø is always false. Hence x[x Ø → x B] is always true. Therefore, Ø is a subset of every set. Note: Set B is always a subset of itself. Subsets 6

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Definition: If A B but A B the we say A is a proper subset of B, denoted A B. Definition: The set of all subset of set A, denoted P(A), is called the power set of A. Subsets 7

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Example: If A = {a, b} then P(A) = {Ø, {a}, {b}, {a,b}} Subsets 8

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Definition: The number of (distinct) elements in A, denoted |A|, is called the cardinality of A. If the cardinality is a natural number (in N), then the set is called finite, else infinite. Subsets 9

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Example: – A = {a, b}, |{a, b}| = 2, |P({a, b})| = 4. – A is finite and so is P(A). – Useful Fact: |A|=n implies |P(A)| = 2 n N is infinite since |N| is not a natural number. It is called a transfinite cardinal number. Note: Sets can be both members and subsets of other sets. Subsets 10

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Example: A = {Ø,{Ø}}. A has two elements and hence four subsets: Ø, {Ø}, {{Ø}}, {Ø,{Ø}} Note that Ø is both a member of A and a subset of A! P. 1 Subsets 11

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Example: A = {Ø}. Which one of the follow is incorrect: (a) Ø A (b) Ø A (c) {Ø} A (d) {Ø} A P. 1 Subsets 12

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Russell's paradox: Let S be the set of all sets which are not members of themselves. Is S a member of itself? Another paradox: Henry is a barber who shaves all people who do not shave themselves. Does Henry shave himself? P. 1 Subsets 13

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Definition: The Cartesian product of A with B, denoted A B, is the set of ordered pairs { | a A Λ b B} Notation: A i ={ |a i A i } Note: The Cartesian product of anything with Ø is Ø. (why?) P. 1 Subsets 14

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Example: – A = {a,b}, B = {1, 2, 3} – AxB = {,,,,, } – What is BxA? AxBxA? If |A| = m and |B| = n, what is |AxB|? P. 1 Subsets 15

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Sets Subset P. 1 Terms 16

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