Section 1.1 Sets, Functions, and Induction

Section 1.1 Sets, Functions, and Induction - Section 1.1:...

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Section 1.1: Preliminaries - Sets, Functions, and Induction In this section, we deal with a few preliminaries. Having a strong foundation will make the rest of the text much easier to digest. First, we discuss some basic set theory. We then discuss functions and some special types of functions. We end with a discussion on proofs by induction. BASICS of SET THEORY Set theory is the language of mathematics. In order for us to effectively communicate theorems and definitions, we require at least a naïve understanding of this topic. Here, the term “naïve” refers to the fact that many of our arguments will involve words including “and,” “or,” “for some,” and “for every” without a rigorous definition of what these terms mean. Definition: A set S is a collection of objects. The objects in a set are called its elements . If x is an element of a set S , we write x S ; otherwise we write x S . We have two conventional ways of denoting a set. Either we list all of its elements between curly brackets, e.g. { } 1,2,3 , or we write a rule for describing the elements of the set, also in curly brackets, e.g. { } |1 3 and is an integer x x x where the vertical bar is read “such that.” Notice that the two examples actually describe the same set. Also note that the order in which terms appear in a set do not matter, e.g. . Definition: Two sets A and B are said to be equal if and only if they contain exactly the same elements. If A and B are equal, we write A B = ; otherwise we write A B . Definition: Let A and B be sets. We say that A is a subset of B , or that A is contained in B , if every element of A is also an element of B , that is, if x A , then x B also. If this is the case, we write A B ; otherwise, we write A B . We consider any set a subset of itself. If A B with A nonempty (see below) and A B , then A is called a proper subset of B . Example: Let { } 1,3,5,7 A = and let { } 1,2,3,4,5,6,7,8,9 B = . Then clearly A B but B A . Example: Let { } 1,1 A = - and let { } 2 | 1 0 B x x = - = . It follows that A B and B A . In fact, we find that A B = . Theorem: Let A and B be sets. Then A B = if and only if A B and B A . Proof Suppose A B = . Then by definition, A and B contain exactly the same elements. Consider some x A . Since A and B contain exactly the same elements, it follows that x B and therefore, A B . A similar argument shows that B A . Conversely, suppose that A B and B A . Then for every x A , it follows that x B ; similarly, for every x B , we have x A . While this in itself would be enough to prove the statement, let us argue by contradiction. Assume that A and B do not have exactly the same elements. Then either A has an element that is not in B or B has an element that is not in A . Suppose without loss of generality that there is an element of A that
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is not an element of B . This contradicts that A B and so our initial assumption that
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This note was uploaded on 12/13/2011 for the course MATH 115 taught by Professor Plotkin during the Spring '08 term at Rutgers.

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Section 1.1 Sets, Functions, and Induction - Section 1.1:...

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