# MATH135 Lecture Notes - Lecture 6: Disjoint Sets, If And Only If, Subset

81 views3 pages MATH 135 - Lecture 6: Subsets, Set Equality, Converse, and iff
Comparing Sets:
Disjoint sets: S and T are said to be disjoint sets when S ∩ T = ϕ
Ex. S = {1, 2, 3}
T = {6, 8, 10}
Sets S and T are disjoint.
Subsets: A set S is called a subset of a set T, and is written S T, when every element
of S belongs to T
Ex. 1 S = {4, 5}
T = {4, 5, 6}
S T
How do we prove S T? → Prove the following implication: if x S, then x T
Ex. 2 Let A = {n : 4 | (∈ ℕ n - 3)} and B = {2k + 1 : k }. Prove A B.∈ ℤ
Solution: Let x A.
Thus, 4 | (x - 3)
x - 3 = 4k for some k ∈ ℤ
x = 4k + 3
x = 4k + 2 + 1
x = 2(2k + 1) + 1
Since k , therefore 2∈ ℤ k + 1 ∈ ℤ
Therefore x B.
A set S is called a superset of a set T, and written S T, if every element of T
belongs to S.
Proper subsets: A set S is called a proper subset of a set T, and written S T, if every
element of S belongs to T and there exists at least one element in T which does not
belong to S.
Using the previous example, is A B? → Yes, there are values in B that are not
in A (ex. 5)
Set Equality:
Saying that two sets S and T are equal, and writing S = T have exactly the same
elements.
Ex. Two sets S and T are equal, written S = T where S and T have exactly the same
elements. How do we prove S = T?
Show that S T and T S⊆ ⊆
x S x T and x T x S
Let S = {-1, 1, 0} and T = {x |∈ ℝ x3 = x}
Show S T:
Let x S. Then x = -1, 1, 0.
13 = 1, (-1)3 = -1, 03 = 0
Therefore x3 = x and S T.
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## Document Summary

Math 135 - lecture 6: subsets, set equality, converse, and iff. Disjoint sets: s and t are said to be disjoint sets when s t = . Subsets: a set s is called a subset of a set t, and is written s. T, when every element of s belongs to t. Prove the following implication: if n - 3)} and b = {2k + 1 : k x. Thus, 4 | (x - 3) x - 3 = 4k for some k x = 4k + 3 x = 4k + 2 + 1 x = 2(2k + 1) + 1. A set s is called a superset of a set t, and written s. T, if every element of t belongs to s. Proper subsets: a set s is called a proper subset of a set t, and written s.