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Chpt 2-functions-seqs v.5

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1. The document discusses various mathematical functions including absolute value, floor/ceiling functions, factorials, modular arithmetic, exponential functions, logarithmic functions, and polynomials. 2. It defines key properties of functions such as one-to-one, onto, bijective, and inverse functions. It also covers function composition and important sequences like geometric and arithmetic progressions. 3. The document provides formulas for summing sequences and common summations like the sum of the first n natural numbers and the sum of the first n odd integers.

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1 Discrete Math CS 2800 Johnnie Baker jbaker@cs.kent.edu Module Topic Basic Structures: Functions and Sequences
Acknowledgement Most of these slides were either created by Professor Bart Selman at Cornell University or else are modifications of his slides 2
Functions Suppose we have: How do you describe the yellow function? What’s a function ? f(x) = -(1/2)x – 1/2 x f(x)
Functions More generally: Definition: Given A and B, nonempty sets, a function f from A to B is an assignment of exactly one element of B to each element of A. We write f(a)=b if b is the element of B assigned by function f to the element a of A. If f is a function from A to B, we write f : AB. Note: Functions are also called mappings or transformations. B
5 Functions A = {Michael, Toby , John , Chris , Brad } B = { Kathy, Carla, Mary} Let f: A  B be defined as f(a) = mother(a). Michael Toby John Chris Brad Kathy Carol Mary A B
6 Functions More generally: f: RR, f(x) = -(1/2)x – 1/2 domain co-domain A - Domain of f B- Co-Domain of f B
7 Functions More formally: a function f : A  B is a subset of AxB where  a  A, ! b  B and <a,b>  f. A B A B a point! a collection of points! Why not?
Functions - image & preimage For any set S  A, image(S) = {b : a  S, f(a) = b} So, image({Michael, Toby}) = {Kathy} image(A) = B - {Carol} image(S) image(John) = {Kathy} pre-image(Kathy) = {John, Toby, Michael} range of f image(A) Michael Toby John Chris Brad Kathy Carol Mary A B
9 Functions - injection A function f: A  B is one-to-one (injective, an injection) if a,b,c, (f(a) = b  f(c) = b)  a = c Not one-to-one Every b  B has at most 1 preimage. Michael Toby John Chris Brad Kathy Carol Mary
10 Functions - surjection A function f: A  B is onto (surjective, a surjection) if b  B, a  A f(a) = b Not onto Every b  B has at least 1 preimage. Michael Toby John Chris Brad Kathy Carol Mary
11 Functions – one-to-one-correspondence or bijection A function f: A  B is bijective if it is one-to-one and onto. Anna Mark John Paul Sarah Carol Jo Martha Dawn Eve Every b  B has exactly 1 preimage. An important implication of this characteristic: The preimage (f-1) is a function! They are invertible. Anna Mark John Paul Sarah Carol Jo Martha Dawn Eve
12 Functions: inverse function Definition: Given f, a one-to-one correspondence from set A to set B, the inverse function of f is the function that assigns to an element b belonging to B the unique element a in A such that f(a)=b. The inverse function is denoted f-1 . f-1 (b)=a, when f(a)=b. B
13 Functions - examples Suppose f: R+  R+, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? yes yes yes This function is invertible.
14 Functions - examples Suppose f: R  R+, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? no yes no This function is not invertible.
15 Functions - examples Suppose f: R  R, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? no no no
Chpt 2-functions-seqs v.5
17 Functions - composition Let f: AB, and g: BC be functions. Then the composition of f and g is: (f o g)(x) = f(g(x)) Note: (f o g) cannot be defined unless the range of g is a subset of the domain of f. “f composed with g”
18 Example: Let f(x) = 2 x +3; g(x) = 3 x + 2; (f o g) (x) = f(3x + 2) = 2 (3 x + 2 ) + 3 = 6 x + 7. (g o f ) (x) = g (2 x + 3) = 3 (2 x + 3) + 2 = 6 x + 11. As this example shows, (f o g) and (g o f) are not necessarily equal – i.e, the composition of functions is not commutative.
19 Note: (f -1 o f) (a) = f -1(f(a)) = f -1(b) = a. (f o f -1) (b) = f (f -1(b)) = f-(a) = b. Therefore (f-1o f ) = IA and (f o f-1) = IB where IA and IB are the identity function on the sets A and B. (f -1) -1= f
20 Some important functions Absolute value: Domain R; Co-Domain = {0}  R+ |x| = x if x ≥0 -x if x < 0 Ex: |-3| = 3; |3| = 3 Floor function (or greatest integer function): Domain = R; Co-Domain = Z x  = largest integer not greater than x Ex: 3.2 = 3; -2.5 =-3
21 Some important functions Ceiling function: Domain = R; Co-Domain = Z x = smallest integer greater than x Ex: 3.2 = 4; -2.5 =-2
≤ ≤ ≤ ≤ + + + + + +
23 Some important functions Factorial function: Domain = Range = N Error on range n! = n (n-1)(n-2) …, 3 x 2 x 1 Ex: 5! = 5 x 4 x 3 x 2 x 1 = 120 Note: 0! = 1 by convention.
24 Some important functions Mod (or remainder): Domain = N x N+ = {(m,n)| m N, n  N+ } Co-domain Range = N m mod n = m - m/n n Ex: 8 mod 3 = 8 - 8/3 3 = 2 57 mod 12 = 9; Note: This function computes the remainder when m is divided by n. The name of this function is an abbreviation of m modulo n, where modulus means with respect to a modulus (size) of n, which is defined to be the remainder when m is divided by n. Note also that this function is an example in which the domain of the function is a 2-tuple.
25 Some important functions: Exponential Function Exponential function: Domain = R+ x R = {(a,x)| a  R+, x  R } Co-domain Range = R+ f(x) = a x Note: a is a positive constant; x varies. Ex: f(n) = a n = a x a …, x a (n times) How do we define f(x) if x is not a positive integer?
26 Some important functions: Exponential function Exponential function: How do we define f(x) if x is not a positive integer? Important properties of exponential functions: (1) a (x+y) = ax ay; (2) a 1 = a (3) a 0 = 1 See: )( ... ; ; 12123 11112 timesnaaa aaaaaaa aaaaaa n      
27 We get: aathereforeaaaaaa aathereforeaaaa athereforeaaaaa bbbbbb       2 1 22 1 2 1 2 1 2 1 2 1 1 )(0 00011 )( 11 1 By similar arguments: mnmnn m mxxxmx kk aaathereforeatimesmaaa aa )()(,)()( 1 1    Note: This determines ax for all x rational. x is irrational by continuity (we’ll skip “details”).
28 Some important functions: Logarithm Function Logarithm base a: Domain = R+ x R = {(a,x)| a  R+, a>1, x  R } Co-domain Range = R y = log a (x)  ay = x Ex: log 2 (8) =3; log 2 (16) =3; 3 < log 2 (15) <4. Key properties of the log function (they follow from those for exponential): 1. log a (1)=0 (because a0 =1) 2. log a (a)=1 (because a1 =a) 3. log a (xy) = log a (x) + log a (x) (similar arguments) 4. log a (xr) = r log a (x) 5. log a (1/x) = - log a (x) (note 1/x = x-1) 6. log b (x) = log a (x) / log a (b)
29 Logarithm Functions Examples: log 2 (1/4)= - log 2 (4)= - 2. log 2 (-4) undefined log 2 (210 35 )= log 2 (210) + log 2 (35 )=10 log 2 (2) + 5log 2 (3 )= = 10 + 5 log 2 (3 )
30 Limit Properties of Log Function 0 )log( lim )log(lim     x x x x x As x gets large, log(x) grows without bound. But x grows MUCH faster than log(x)…more soon on growth rates.
31 Some important functions: Polynomials Polynomial function: Domain = usually R Co-domain Range = usually R Pn(x) = anxn + an-1xn-1 + … + a1x1 + a0 n, a nonnegative integer is the degree of the polynomial; an 0 (so that the term anxn actually appears) (an, an-1, …, a1, a0) are the coefficients of the polynomial. Ex: y = P1(x) = a1x1 + a0 linear function y = P2(x) = a2x2 + a1x1 + a0 quadratic polynomial or function
32 Exponentials grow MUCH faster than polynomials: 10lim 0    bif b xaa x k k x  We’ll talk more about growth rates in the next module….
Sequences
34 Sequences Definition: A sequence {ai} is a function f: A  N  {0}  S, where we write ai to indicate f(i). We call ai term I of the sequence. Examples: Sequence {ai}, where ai = i is just a0 = 0, a1 = 1, a2 = 2, … Sequence {ai}, where ai = i2 is just a0 = 0, a1 = 1, a2 = 4, … Sequences of the form a1, a2, …, an are often used in computer science. (always check whether sequence starts at a0 or a1) These finite sequences are also called strings. The length of a string is the number of terms in the string. The empty string, denoted by , is the string that has no terms.
35 Geometric and Arithmetic Progressions Definition: A geometric progression is a sequence of the form  ,,,,,, 32 n arararara The initial term a and the common ratio r are real numbers Definition: An arithmetic progression is a sequence of the form  ,,,3,2,, ndadadadaa  The initial term a and the common difference d are real numbers Note: An arithmetic progression is a discrete analogue of the linear function f(x) = dx + a
Notice differences in growth rate.
37 Summation The symbol  (Greek letter sigma) is used to denote summation. The limit: ai i1 k   a1  a2  K  ak ai i1    lim n ai i1 n  i is the index of the summation, and the choice of letter i is arbitrary; the index of the summation runs through all integers, with its lower limit 1 and ending upper limit k.
38 Summation The laws for arithmetic apply to summations  cai  bi  i1 k   c ai i1 k   bi i1 k  Use associativity to separate the b terms from the a terms. Use distributivity to factor the c’s.
39 Summations you should know… What is S = 1 + 2 + 3 + … + n? You get n copies of (n+1). But we’ve over added by a factor of 2. So just divide by 2. S = 1 + 2 + … + n S = n + n-1 + … + 1 2s = n+1 + n+1 + … + n+1 Write the sum. Write it again. Add together. k k1 n   n(n  1) 2 (little) Gauss in 4th grade.  Why whole number?
40 What is S = 1 + 3 + 5 + … + (2n - 1)? Sum of first n odds.  (2k 1) k1 n   2 k k1 n   1 k1 n   2 n(n  1) 2       n  n2
What is S = 1 + 3 + 5 + … + (2n - 1)? Sum of first n odds.   n2
What is S = 1 + r + r2 + … + rn Geometric Series  rk k 0 n   1 r  K  rn  r rk k 0 n   r  r2  K  rn1 Multiply by r Subtract the summations rk k 0 n   r rk k 0 n   1 rn1 factor (1 r) rk k 0 n   1 rn1 divide rk k 0 n   1 rn 1 (1 r) DONE!
What about: rk k 0    1 r  K  rn  K  n lim 1 rn1 (1 r) If r  1 this blows up. If r < 1 we can say something. rk k 0    n lim rk k 0 n   1 (1 r) Try r = ½.
Useful Summations
45 Infinite Cardinality How can we extend the notion of cardinality to infinite sets? Definition: Two sets A and B have the same cardinality if and only if there exists a bijection (or a one-to-one correspondence) between them, A ~ B. We split infinite sets into two groups: 1. Sets with the same cardinality as the set of natural numbers 2. Sets with different cardinality as the set of natural numbers
46 Infinite Cardinality Definition: A set is countable if it is finite or has the same cardinality as the set of positive integers. Definition: A set is uncountable if it is not countable. Definition: The cardinality of an infinite set S that is countable is denotes by ‫א‬0 (where ‫א‬ is aleph, the first letter of the Hebrew alphabet). We write |S| = ‫א‬0 and say that S has cardinality “aleph null”. Note: Georg Cantor defined the notion of cardinality and was the first to realize that infinite sets can have different cardinalities. ‫א‬0 is the cardinality of the natural numbers; the next larger cardinality is aleph-one ‫א‬1, then, ‫א‬2 and so on.
47 Infinite Cardinality: Odd Positive Integers Example: The set of odd positive integers is a countable set. Let’s define the function f, from Z+ to the set of odd positive numbers, f(n) = 2 n -1 We have to show that f is both one-to-one and onto. a) one-to-one Suppose f(n)= f(m)  2n-1 = 2m-1  n=m b) onto Suppose that t is an odd positive integer. Then t is 1 less than an even integer 2k, where k is a natural number. hence t=2k-1= f(k).
48 Infinite Cardinality: Odd Positive Integers 2
49 Infinite Cardinality: Integers Example: The set of integers is a countable set. Lets consider the sequence of all integers, starting with 0: 0,1,-1,2,- 2,…. We can define this sequence as a function: oddNn n evenNnn , 2 )1( , 2    f(n) = Show at home that it’s one-to-one and onto 20 1 -1 2
50 Infinite Cardinality: Rational Numbers Example: The set of positive rational numbers is a countable set. Hmm…
51 Infinite Cardinality: Rational Numbers Example: The set of positive rational numbers is a countable set Key aspect to list the rational numbers as a sequence – every positive number is the quotient p/q of two positive integers. Visualization of the proof. p+q=4 p+q=5 p+q=6 Since all positive rational numbers are listed once, the set of positive rational numbers is countable.
52 Uncountable Sets: Cantor's diagonal argument The set of all infinite sequences of zeros and ones is uncountable. Consider a sequence, 10,,,,, 21  iin aoranaaa  For example: So in general we have: i.e., sn,m is the mth element of the nth sequence on the list.
53 It is possible to build a sequence, say s0, in such a way that its first element is different from the first element of the first sequence in the list, its second element is different from the second element of the second sequence in the list, and, in general, its nth element is different from the nth element of the nth sequence in the list. In other words, s0,m will be 0 if sm,m is 1, and s0,m will be 1 if sm,m is 0. Uncountable Sets: Cantor's diagonal argument
54 The sequence s0 is distinct from all the sequences in the list. Why? Let’s say that s0 is identical to the 100th sequence, therefore, s0,100=s100,100. In general, if it appeared as the nth sequence on the list, we would have s0,n = sn,n, which, due to the construction of s0, is impossible. Note: the diagonal elements are highlighted, showing why this is called the diagonal argument Uncountable Sets: Cantor's diagonal argument
55 From this it follows that the set T, consisting of all infinite sequences of zeros and ones, cannot be put into a list s1, s2, s3, ... Otherwise, it would be possible by the above process to construct a sequence s0 which would both be in T (because it is a sequence of 0's and 1's which is by the definition of T in T) and at the same time not in T (because we can deliberately construct it not to be in the list). T, containing all such sequences, must contain s0, which is just such a sequence. But since s0 does not appear anywhere on the list, T cannot contain s0. Therefore T cannot be placed in one-to-one correspondence with the natural numbers. In other words, the set of infinite binary strings is uncountable. Uncountable Sets: Cantor's diagonal argument
56 Real Numbers Example; The set of real numbers is an uncountable set. Let’s assume that the set of real numbers is countable. Therefore any subset of it is also countable, in particular the interval [0,1]. How many real numbers are in interval [0, 1]?
57 Real Numbers How many real numbers are in interval [0, 1]? 0.4 3 2 9 0 1 3 2 9 8 4 2 0 3 9 … 0.8 2 5 9 9 1 3 2 7 2 5 8 9 2 5 … 0.9 2 5 3 9 1 5 9 7 4 5 0 6 2 1 … … “Countably many! There’s part of the list!” “Are you sure they’re all there?” 0.5 3 6 … So we say the reals are “uncountable.” Counterexample: Use diagonalization to create a new number that differs in the ith position of the ith number by 1.

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Chpt 2-functions-seqs v.5

  • 1. 1 Discrete Math CS 2800 Johnnie Baker jbaker@cs.kent.edu Module Topic Basic Structures: Functions and Sequences
  • 2. Acknowledgement Most of these slides were either created by Professor Bart Selman at Cornell University or else are modifications of his slides 2
  • 3. Functions Suppose we have: How do you describe the yellow function? What’s a function ? f(x) = -(1/2)x – 1/2 x f(x)
  • 4. Functions More generally: Definition: Given A and B, nonempty sets, a function f from A to B is an assignment of exactly one element of B to each element of A. We write f(a)=b if b is the element of B assigned by function f to the element a of A. If f is a function from A to B, we write f : AB. Note: Functions are also called mappings or transformations. B
  • 5. 5 Functions A = {Michael, Toby , John , Chris , Brad } B = { Kathy, Carla, Mary} Let f: A  B be defined as f(a) = mother(a). Michael Toby John Chris Brad Kathy Carol Mary A B
  • 6. 6 Functions More generally: f: RR, f(x) = -(1/2)x – 1/2 domain co-domain A - Domain of f B- Co-Domain of f B
  • 7. 7 Functions More formally: a function f : A  B is a subset of AxB where  a  A, ! b  B and <a,b>  f. A B A B a point! a collection of points! Why not?
  • 8. Functions - image & preimage For any set S  A, image(S) = {b : a  S, f(a) = b} So, image({Michael, Toby}) = {Kathy} image(A) = B - {Carol} image(S) image(John) = {Kathy} pre-image(Kathy) = {John, Toby, Michael} range of f image(A) Michael Toby John Chris Brad Kathy Carol Mary A B
  • 9. 9 Functions - injection A function f: A  B is one-to-one (injective, an injection) if a,b,c, (f(a) = b  f(c) = b)  a = c Not one-to-one Every b  B has at most 1 preimage. Michael Toby John Chris Brad Kathy Carol Mary
  • 10. 10 Functions - surjection A function f: A  B is onto (surjective, a surjection) if b  B, a  A f(a) = b Not onto Every b  B has at least 1 preimage. Michael Toby John Chris Brad Kathy Carol Mary
  • 11. 11 Functions – one-to-one-correspondence or bijection A function f: A  B is bijective if it is one-to-one and onto. Anna Mark John Paul Sarah Carol Jo Martha Dawn Eve Every b  B has exactly 1 preimage. An important implication of this characteristic: The preimage (f-1) is a function! They are invertible. Anna Mark John Paul Sarah Carol Jo Martha Dawn Eve
  • 12. 12 Functions: inverse function Definition: Given f, a one-to-one correspondence from set A to set B, the inverse function of f is the function that assigns to an element b belonging to B the unique element a in A such that f(a)=b. The inverse function is denoted f-1 . f-1 (b)=a, when f(a)=b. B
  • 13. 13 Functions - examples Suppose f: R+  R+, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? yes yes yes This function is invertible.
  • 14. 14 Functions - examples Suppose f: R  R+, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? no yes no This function is not invertible.
  • 15. 15 Functions - examples Suppose f: R  R, f(x) = x2. Is f one-to-one? Is f onto? Is f bijective? no no no
  • 17. 17 Functions - composition Let f: AB, and g: BC be functions. Then the composition of f and g is: (f o g)(x) = f(g(x)) Note: (f o g) cannot be defined unless the range of g is a subset of the domain of f. “f composed with g”
  • 18. 18 Example: Let f(x) = 2 x +3; g(x) = 3 x + 2; (f o g) (x) = f(3x + 2) = 2 (3 x + 2 ) + 3 = 6 x + 7. (g o f ) (x) = g (2 x + 3) = 3 (2 x + 3) + 2 = 6 x + 11. As this example shows, (f o g) and (g o f) are not necessarily equal – i.e, the composition of functions is not commutative.
  • 19. 19 Note: (f -1 o f) (a) = f -1(f(a)) = f -1(b) = a. (f o f -1) (b) = f (f -1(b)) = f-(a) = b. Therefore (f-1o f ) = IA and (f o f-1) = IB where IA and IB are the identity function on the sets A and B. (f -1) -1= f
  • 20. 20 Some important functions Absolute value: Domain R; Co-Domain = {0}  R+ |x| = x if x ≥0 -x if x < 0 Ex: |-3| = 3; |3| = 3 Floor function (or greatest integer function): Domain = R; Co-Domain = Z x  = largest integer not greater than x Ex: 3.2 = 3; -2.5 =-3
  • 21. 21 Some important functions Ceiling function: Domain = R; Co-Domain = Z x = smallest integer greater than x Ex: 3.2 = 4; -2.5 =-2
  • 23. 23 Some important functions Factorial function: Domain = Range = N Error on range n! = n (n-1)(n-2) …, 3 x 2 x 1 Ex: 5! = 5 x 4 x 3 x 2 x 1 = 120 Note: 0! = 1 by convention.
  • 24. 24 Some important functions Mod (or remainder): Domain = N x N+ = {(m,n)| m N, n  N+ } Co-domain Range = N m mod n = m - m/n n Ex: 8 mod 3 = 8 - 8/3 3 = 2 57 mod 12 = 9; Note: This function computes the remainder when m is divided by n. The name of this function is an abbreviation of m modulo n, where modulus means with respect to a modulus (size) of n, which is defined to be the remainder when m is divided by n. Note also that this function is an example in which the domain of the function is a 2-tuple.
  • 25. 25 Some important functions: Exponential Function Exponential function: Domain = R+ x R = {(a,x)| a  R+, x  R } Co-domain Range = R+ f(x) = a x Note: a is a positive constant; x varies. Ex: f(n) = a n = a x a …, x a (n times) How do we define f(x) if x is not a positive integer?
  • 26. 26 Some important functions: Exponential function Exponential function: How do we define f(x) if x is not a positive integer? Important properties of exponential functions: (1) a (x+y) = ax ay; (2) a 1 = a (3) a 0 = 1 See: )( ... ; ; 12123 11112 timesnaaa aaaaaaa aaaaaa n      
  • 27. 27 We get: aathereforeaaaaaa aathereforeaaaa athereforeaaaaa bbbbbb       2 1 22 1 2 1 2 1 2 1 2 1 1 )(0 00011 )( 11 1 By similar arguments: mnmnn m mxxxmx kk aaathereforeatimesmaaa aa )()(,)()( 1 1    Note: This determines ax for all x rational. x is irrational by continuity (we’ll skip “details”).
  • 28. 28 Some important functions: Logarithm Function Logarithm base a: Domain = R+ x R = {(a,x)| a  R+, a>1, x  R } Co-domain Range = R y = log a (x)  ay = x Ex: log 2 (8) =3; log 2 (16) =3; 3 < log 2 (15) <4. Key properties of the log function (they follow from those for exponential): 1. log a (1)=0 (because a0 =1) 2. log a (a)=1 (because a1 =a) 3. log a (xy) = log a (x) + log a (x) (similar arguments) 4. log a (xr) = r log a (x) 5. log a (1/x) = - log a (x) (note 1/x = x-1) 6. log b (x) = log a (x) / log a (b)
  • 29. 29 Logarithm Functions Examples: log 2 (1/4)= - log 2 (4)= - 2. log 2 (-4) undefined log 2 (210 35 )= log 2 (210) + log 2 (35 )=10 log 2 (2) + 5log 2 (3 )= = 10 + 5 log 2 (3 )
  • 30. 30 Limit Properties of Log Function 0 )log( lim )log(lim     x x x x x As x gets large, log(x) grows without bound. But x grows MUCH faster than log(x)…more soon on growth rates.
  • 31. 31 Some important functions: Polynomials Polynomial function: Domain = usually R Co-domain Range = usually R Pn(x) = anxn + an-1xn-1 + … + a1x1 + a0 n, a nonnegative integer is the degree of the polynomial; an 0 (so that the term anxn actually appears) (an, an-1, …, a1, a0) are the coefficients of the polynomial. Ex: y = P1(x) = a1x1 + a0 linear function y = P2(x) = a2x2 + a1x1 + a0 quadratic polynomial or function
  • 32. 32 Exponentials grow MUCH faster than polynomials: 10lim 0    bif b xaa x k k x  We’ll talk more about growth rates in the next module….
  • 34. 34 Sequences Definition: A sequence {ai} is a function f: A  N  {0}  S, where we write ai to indicate f(i). We call ai term I of the sequence. Examples: Sequence {ai}, where ai = i is just a0 = 0, a1 = 1, a2 = 2, … Sequence {ai}, where ai = i2 is just a0 = 0, a1 = 1, a2 = 4, … Sequences of the form a1, a2, …, an are often used in computer science. (always check whether sequence starts at a0 or a1) These finite sequences are also called strings. The length of a string is the number of terms in the string. The empty string, denoted by , is the string that has no terms.
  • 35. 35 Geometric and Arithmetic Progressions Definition: A geometric progression is a sequence of the form  ,,,,,, 32 n arararara The initial term a and the common ratio r are real numbers Definition: An arithmetic progression is a sequence of the form  ,,,3,2,, ndadadadaa  The initial term a and the common difference d are real numbers Note: An arithmetic progression is a discrete analogue of the linear function f(x) = dx + a
  • 36. Notice differences in growth rate.
  • 37. 37 Summation The symbol  (Greek letter sigma) is used to denote summation. The limit: ai i1 k   a1  a2  K  ak ai i1    lim n ai i1 n  i is the index of the summation, and the choice of letter i is arbitrary; the index of the summation runs through all integers, with its lower limit 1 and ending upper limit k.
  • 38. 38 Summation The laws for arithmetic apply to summations  cai  bi  i1 k   c ai i1 k   bi i1 k  Use associativity to separate the b terms from the a terms. Use distributivity to factor the c’s.
  • 39. 39 Summations you should know… What is S = 1 + 2 + 3 + … + n? You get n copies of (n+1). But we’ve over added by a factor of 2. So just divide by 2. S = 1 + 2 + … + n S = n + n-1 + … + 1 2s = n+1 + n+1 + … + n+1 Write the sum. Write it again. Add together. k k1 n   n(n  1) 2 (little) Gauss in 4th grade.  Why whole number?
  • 40. 40 What is S = 1 + 3 + 5 + … + (2n - 1)? Sum of first n odds.  (2k 1) k1 n   2 k k1 n   1 k1 n   2 n(n  1) 2       n  n2
  • 41. What is S = 1 + 3 + 5 + … + (2n - 1)? Sum of first n odds.   n2
  • 42. What is S = 1 + r + r2 + … + rn Geometric Series  rk k 0 n   1 r  K  rn  r rk k 0 n   r  r2  K  rn1 Multiply by r Subtract the summations rk k 0 n   r rk k 0 n   1 rn1 factor (1 r) rk k 0 n   1 rn1 divide rk k 0 n   1 rn 1 (1 r) DONE!
  • 43. What about: rk k 0    1 r  K  rn  K  n lim 1 rn1 (1 r) If r  1 this blows up. If r < 1 we can say something. rk k 0    n lim rk k 0 n   1 (1 r) Try r = ½.
  • 45. 45 Infinite Cardinality How can we extend the notion of cardinality to infinite sets? Definition: Two sets A and B have the same cardinality if and only if there exists a bijection (or a one-to-one correspondence) between them, A ~ B. We split infinite sets into two groups: 1. Sets with the same cardinality as the set of natural numbers 2. Sets with different cardinality as the set of natural numbers
  • 46. 46 Infinite Cardinality Definition: A set is countable if it is finite or has the same cardinality as the set of positive integers. Definition: A set is uncountable if it is not countable. Definition: The cardinality of an infinite set S that is countable is denotes by ‫א‬0 (where ‫א‬ is aleph, the first letter of the Hebrew alphabet). We write |S| = ‫א‬0 and say that S has cardinality “aleph null”. Note: Georg Cantor defined the notion of cardinality and was the first to realize that infinite sets can have different cardinalities. ‫א‬0 is the cardinality of the natural numbers; the next larger cardinality is aleph-one ‫א‬1, then, ‫א‬2 and so on.
  • 47. 47 Infinite Cardinality: Odd Positive Integers Example: The set of odd positive integers is a countable set. Let’s define the function f, from Z+ to the set of odd positive numbers, f(n) = 2 n -1 We have to show that f is both one-to-one and onto. a) one-to-one Suppose f(n)= f(m)  2n-1 = 2m-1  n=m b) onto Suppose that t is an odd positive integer. Then t is 1 less than an even integer 2k, where k is a natural number. hence t=2k-1= f(k).
  • 49. 49 Infinite Cardinality: Integers Example: The set of integers is a countable set. Lets consider the sequence of all integers, starting with 0: 0,1,-1,2,- 2,…. We can define this sequence as a function: oddNn n evenNnn , 2 )1( , 2    f(n) = Show at home that it’s one-to-one and onto 20 1 -1 2
  • 50. 50 Infinite Cardinality: Rational Numbers Example: The set of positive rational numbers is a countable set. Hmm…
  • 51. 51 Infinite Cardinality: Rational Numbers Example: The set of positive rational numbers is a countable set Key aspect to list the rational numbers as a sequence – every positive number is the quotient p/q of two positive integers. Visualization of the proof. p+q=4 p+q=5 p+q=6 Since all positive rational numbers are listed once, the set of positive rational numbers is countable.
  • 52. 52 Uncountable Sets: Cantor's diagonal argument The set of all infinite sequences of zeros and ones is uncountable. Consider a sequence, 10,,,,, 21  iin aoranaaa  For example: So in general we have: i.e., sn,m is the mth element of the nth sequence on the list.
  • 53. 53 It is possible to build a sequence, say s0, in such a way that its first element is different from the first element of the first sequence in the list, its second element is different from the second element of the second sequence in the list, and, in general, its nth element is different from the nth element of the nth sequence in the list. In other words, s0,m will be 0 if sm,m is 1, and s0,m will be 1 if sm,m is 0. Uncountable Sets: Cantor's diagonal argument
  • 54. 54 The sequence s0 is distinct from all the sequences in the list. Why? Let’s say that s0 is identical to the 100th sequence, therefore, s0,100=s100,100. In general, if it appeared as the nth sequence on the list, we would have s0,n = sn,n, which, due to the construction of s0, is impossible. Note: the diagonal elements are highlighted, showing why this is called the diagonal argument Uncountable Sets: Cantor's diagonal argument
  • 55. 55 From this it follows that the set T, consisting of all infinite sequences of zeros and ones, cannot be put into a list s1, s2, s3, ... Otherwise, it would be possible by the above process to construct a sequence s0 which would both be in T (because it is a sequence of 0's and 1's which is by the definition of T in T) and at the same time not in T (because we can deliberately construct it not to be in the list). T, containing all such sequences, must contain s0, which is just such a sequence. But since s0 does not appear anywhere on the list, T cannot contain s0. Therefore T cannot be placed in one-to-one correspondence with the natural numbers. In other words, the set of infinite binary strings is uncountable. Uncountable Sets: Cantor's diagonal argument
  • 56. 56 Real Numbers Example; The set of real numbers is an uncountable set. Let’s assume that the set of real numbers is countable. Therefore any subset of it is also countable, in particular the interval [0,1]. How many real numbers are in interval [0, 1]?
  • 57. 57 Real Numbers How many real numbers are in interval [0, 1]? 0.4 3 2 9 0 1 3 2 9 8 4 2 0 3 9 … 0.8 2 5 9 9 1 3 2 7 2 5 8 9 2 5 … 0.9 2 5 3 9 1 5 9 7 4 5 0 6 2 1 … … “Countably many! There’s part of the list!” “Are you sure they’re all there?” 0.5 3 6 … So we say the reals are “uncountable.” Counterexample: Use diagonalization to create a new number that differs in the ith position of the ith number by 1.