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# Class Note for MATH 300 at UMass(1)

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Date Created: 02/06/15

Math 300 Fall 2008 Note 6 Zhigang Han Umass at Amherst 1 Functions and Bijections 11 Functions Def 1 Let X and Y be sets A function 1 from X to Y denoted by f X a Y is a rule that assigns to each element x E X a unique element y E Y We write y x and call it the value of the function f at x The set X is called the domain of f and Y is called the codomain of f The set of values of f is called the image of f and is denoted by imf That is imf y E Yly x for some x E X Note that imf is a subset of the codomain Y Rmk ln calculus we often write 1 R a R such that x xxi 2 Note that by our de nition 1 is not a function since the domain for f is not equal to R However if we denote X x 2 2 then f X a R is a function Note also that it s OK to keep the codomain as R although 11110 yly Z 0 Def 2 Two functions f and g are equal if they have the same domain the same codomain and x gx for all x in the domain Eg 1 Let f R a R be a function such that x x2 and g ZH Z be another function such that gx x2 1 and g are not equal since they have different domain and different codomain Eg 2 Let f and G be functions from N to N de ned by n lcm2n and gn 3 7 71 712 Then 1 and g are equal since they have the same domain and the same codomain and n gn for all n E N Check this Def 3 Let f X a Y and g Y a Z are two functions such that the codomain of f is equal to the domain of 9 De ne the composite of f and g7 denoted by g o f as the function with domain X and codomain Z such that 9 O f 90 Eg 3 Let X be the set of nonnegative numbers Let f X a R be de ned by x Let g R a R be de ned by gz 2x 1 Are the composite f o g and g o f de ned Theorem Associativity Let f X a Y7 g Y a Z and h Z a W be three functions Then hogof homo Xew Rmk Because of the associativity7 it makes sense to write the composite of three functions as h o g o f Def 4 The identity function on X7 denoted by 1X7 is de ned to be the function 1X X a X such that 1Xm z for all zEX Prop 1 Let f XHYbeafunctionThenf01X f and 1yof f Def 5 Let f X HY and g Y a X be two functions If fog 1X and 901 1Y7 then 9 is called an inverse of f and f is called an inverse of 9 We write 9 f 1 and f g l Prop 2 Uniqueness of the inverse If a function f X a Y has an inverse7 then the inverse is unique Proof Assume f has two inverses g and h By the de nition of inverse7 g and h both have the domain Y and codomain X Also from the de nition of inverse we have 9 o f 1X and f o h 1y We also have the other two equalities but we don t need them for our proof F romgof1Xwegetgofoh1Xohhandfrom foh1ywe get 9 o f o h g 0 1y 9 Hence 9 h This proves the uniqueness Rink Since the inverse is unique we can write 9 f 1 and also 1 g l We also have f 1 of 1X and fo f l 1y Rink about interchanging z and y There s no need to interchange z and y especially when the variables x and y represent have their speci c meanings Eg 4 Let f R a R and g R a R be two functions de ned by x 3 1 and gy y 7 Show that g is the inverse function of f 12 The Inversion Theorem Def 6 A function f X a Y is called injective or onetoone if HM Hm x1 m2 That means distinct elements in the domain must have distinct images An injective function is called an injection Def 7 A function f X a Y is called surjective or onto if 1mm Y That means the image is the whole of the codomain An surjective function is called an surjection Def 8 A function f X a Y is called bijective or onetoone cor respondence if it is both injective and surjective A bijective function is called a bijection Rink 1 A function is injective if for each y E Y there is at most one element x E X such that y 2 A function is surjective if for each y E Y there is at least one element x E X such that y 3 A function is bijective if 7 for each y 6 Y7 there is precisely one element x E X such that y Inversion Theorem A function has an inverse if and only if it is bijective Eg 5 Does the function f R a R de ned by x 5 7 z have an inverse Prop 3 Let f X a Y and g Y a Z be two functions 1 If f and g are both injections7 then 9 o f is an injection 2 If f and g are both surjections7 then 9 o f is an surjection 3 If f and g are both loijections7 then 9 o f is an bijection 13 Cardinality Def 9 We say a set X has the same cardinality as a set Y7 and we write le lYl7 if there exists a bijection from X to Y We write le 7 lYl ifX does not have the same cardinality as Y Rmk le lYl implies lYl le since if there exists a bijection 1 from X to Y7 then there exists a bijection from Y to X7 namely the inverse of 1 From now on7 it makes sense to say that X and Y have the same cardinality Def 10 We say the cardinality of a set Y is greater than or equal to the cardinality of a set X7 and we write lYl 2 le or le lYl7 if there exists an injection from X to Y Def 11 We say the cardinality of a set Y is strictly greater than the cardinality ofa set X7 and we write lYl gt le or le lt lYl7 if lYl 2 le and le 7 Rmk To show lYl gt le one needs to show there exists an injection from X to Y which is often easy and by a direct construction7 but there exists no bijection from X to Y which is usually harder and by contradiction Eg 6 De ne a relation R on the set A of all sets such that ARB if and only if A has the same cardinality as B Show that R is an equivalence relation Eg 7 Show that ifA Q B7 then lAl ls it true that ifA C B7 then 1A1 lt 1317 Prop 4 HA and B are nite sets7 that is7 sets with nitely many elements7 then lAl B if and only if A and B have the same number of elements Rmk That s why we often write X Y for le Prop 5 l2 Prop 6 Q Theorem R gt Rmk This theorem was proved by Georg Cantor in 1874 The proof is called Cantor7s diagonal argument Def 12 A set is said to be countable if it s nite or it has the same cardinality as N is uncountable otherwise Rmk So N Z and Q are countable7 while R is uncountable Eg 8 Show that A mlO x g 1 and B yl3 y g 5 have the same cardinality Eg 9 Prove that any two closed intervals have the same cardinality Eg 10 Prove that A 0 lt z lt 1 and R have the same cardinality Eg 11 Prove that any two open intervals have the same cardinality A Challenge Problem Show that A mlO lt z lt 1 and B le y g 1 have the same cardinality

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