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# Discrete Mathematics Lecture 7 PowerPoint PPT Presentation

Discrete Mathematics Lecture 7. Alexander Bukharovich New York University. Generic Functions. A function f: X  Y is a relationship between elements of X to elements of Y, when each element from X is related to a unique element from Y

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Discrete Mathematics Lecture 7

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Discrete MathematicsLecture 7

Alexander Bukharovich

New York University

### Generic Functions

• A function f: X  Y is a relationship between elements of X to elements of Y, when each element from X is related to a unique element from Y

• X is called domain of f, range of f is a subset of Y so that for each element y of this subset there exists an element x from X such that y = f(x)

• Sample functions:

• f : R  R, f(x) = x2

• f : Z  Z, f(x) = x + 1

• f : Q  Z, f(x) = 2

### Generic Functions

• Arrow diagrams for functions

• Non-functions

• Equality of functions:

• f(x) = |x| and g(x) = sqrt(x2)

• Identity function

• Sequences as functions

• Functions with a domain of some language

• Logarithmic function

### Generic Functions

• Encoding and decoding of characters

• Hamming distance function: number of differences between two encodings

• Boolean functions: f : {0, 1}n {0, 1}

• Well-defined functions

• Euler function: (n) is the number of positive integers less than n, which are mutually prime with n

### Exercises

• Show that if p is a prime number, then (pn) = pn – pn-1

• Prove that there infinitely many integers for which Euler’s function is a perfect square

• Show that (pq) = (p-1) (q-1) if p and q are distinct primes

• Determine which of the following is true:

• if A  B, then f(A)  f(B)

• f(A  B) = f(A)  f(B)

• f(A  B) = f(A)  f(B)

• f(A - B) = f(A) - f(B)

### Finite-State Automata (FSA)

• Finite-state automata A is defined by 5 objects:

• Set I of the input alphabet

• Set S of automaton states

• Designated initial state s0 from S

• Designated set of accepted states from S

• Next-state function N: S  I  S that associates next state to the pair {current state, input symbol}

• Descriptions of finite-state automaton:

• State-transition diagram

• Next-state table

1

1

s0

s1

0

s2

0

0

1

### FSA and Languages

• Let A be an FSA with an input alphabet I. The set of all strings w from I* such that A goes to accepting state on w is called a language accepted by A: L(A)

• Eventual state-function N* : S  I*  S is a function that maps a pair {state, input string} to the state to which FSA would lead from the original state given the symbols in the input string as an input.

### Designing FSA

• Design an FSA that accepts all strings of 0’s and 1’s such that the number of 1’s is divisible by 3

• Design an FSA that accepts the set of strings that contain exactly one 1

• Design an FSA with alphabet {a, b} which accepts strings that end on the same two characters

• Simulating an FSA using software

### One-to-One Functions

• Function f : X  Y is called one-to-one (injective) when for all elements x1 and x2 from X if f(x1) = f(x2), then x1 = x2

• Determine whether the following functions are one-to-one:

• f : R  R, f(x) = 4x – 1

• g : Z  Z, g(n) = n2

• Hash functions

### Onto Functions

• Function f : X  Y is called onto (surjective) when given any element y from Y, there exists x in X so that f(x) = y

• Determine whether the following functions are onto:

• f : R  R, f(x) = 4x – 1

• f : Z  Z, g(n) = 4n – 1

• Bijection is one-to-one and onto

• Reversing strings function is bijective

### Inverse Functions

• If f : X  Y is a bijective function, then it is possible to define an inverse function f-1: Y  X so that f-1(y) = x whenever f(x) = y

• Find an inverse for the following functions:

• String-reverse function

• f : R  R, f(x) = 4x – 1

• Inverse function of a bijective function is a bijective function itself

### Exercises

• Let cm,n be the number of onto functions from a set of m elements to a set of n elements. Find a relationship between cm,n, cm-1,n and cm-1,n-1

• Let F: Z  Z  Z and G: Z  Z  Z, F(n, m) = 3n6m and G(n, m) = 3n5m. Is F one-to-one, is G one-to-one?

### Pigeonhole Principle

• If n pigeons fly into m pigeonholes and n > m, then at least one hole must contain two or more pigeons

• A function from one finite set to a smaller finite set cannot be one-to-one

• In a group of 13 people must there be at least two who have birthday in the same month?

• A drawer contains 10 black and 10 white socks. How many socks need to be picked to ensure that a pair is found?

• Let A = {1, 2, 3, 4, 5, 6, 7, 8}. If 5 integers are selected must at least one pair have sum of 9?

### Pigeonhole Principle

• There is no FSA that accepts the following language: L = {s = akbk, for positive k}

• Generalized Pigeonhole Principle: For any function f : X  Y acting on finite sets, if n(X) > k * N(Y), then there exists some y from Y so that there are at least k + 1 distinct x’s so that f(x) = y

• There are 42 students who are to share 12 computers. Each student uses exactly 1 computer and no computer is used by more than 6 students. Show that at least 5 computers are used by 3 or more students.

### Exercises

• Let f : X  Y and n(X) = n(Y), then f is bijective iff f is surjective

• Let A be a set of 6 integers less than 13. Show that there must be two disjoint subsets of A whose sum of elements adds up to the same number

• Given 52 distinct integers, show that there must be two whose sum or difference is divisible by 100

• Show that if 101 integers are chosen from 1 to 200 inclusive, there must be two with the property that one is divisible by the other

• Suppose a1, a2, …, an is a sequence of n integers none of which is divisible by n. Show that at least one difference ai – aj is divisible by n

### Composition of Functions

• Let f : X  Y and g : Y  Z, let range of f be a subset of the domain of g. The we can define a composition of g o f : X  Z

• Let f,g : Z  Z, f(n) = n + 1, g(n) = n2. Find f o g and g o f.

• Composition with identity function

• Composition with an inverse function

• Composition of two one-to-one functions is one-to-one

• Composition of two onto functions is onto

### Cardinality

• Cardinality refers to the size of the set

• Finite and infinite sets

• Two sets have the same cardinality when there is bijective function associating them

• Cardinality is is reflexive, symmetric and transitive

• Countable sets: set of all integers, set of even numbers

• Set of real numbers

• Computability of functions

### Exercises

• Show that the set of irrational numbers is dense

• Show that a power set has always a greater cardinality than the original set