Snick snack CPSC 121: Models of Computation 2008/9 Winter Term 2 Functions Steve Wolfman, based on notes by Patrice Belleville and others

snick

snack

CPSC 121: Models of Computation2008/9 Winter Term 2

Functions

Steve Wolfman, based on notes by Patrice Belleville and others

Lecture Prerequisites

Read Section 7.1.

Solve problems like Exercise Set 7.1, #1-4, 13-14, 23-24.

Learning Goals: Pre-Class

By the start of class, you should be able to:– Define the terms domain, co-domain, range,

image, and pre-image– Use appropriate function syntax to relate

these terms (e.g., f : A B indicates that f is a function mapping domain A to co-domain B).

– Determine whether f : A B is a function given a definition for f as an equation or arrow diagram.

Learning Goals: In-Class

By the end of this unit, you should be able to:– Define the terms injective (one-to-one),

surjective (onto), bijective (one-to-one correspondence), and inverse.

– Determine whether a given function is injective, surjective, and/or bijective.

– Apply your proof skills to proofs about the properties (e.g., injectiveness, surjectiveness, bijectiveness, and function-ness) of functions and their inverses.

Outline

• Functions as “Computation Abstractions”• Definition of Functions and

Function Terminology• Function Properties

– Injective– Surjective– Bijective

• Function Operations– Inverse– Composition Dropped from learning goals and exam.

What is this?

control

data1

data2

output

What is this?

control

data1

data2

output

What is this?

control

data1

data2

output

What is this?

control

data1

data2output

In the lab, you implemented a multiplexer and then used it as a piece in larger circuits.

You abstracted from the concrete implementation to a description of its function:

f(control, data1, data2) = output= (~control data1) (control data2) = data1 if control is 0, but data2 otherwise

Functions, Abstraction, and CPSC 121

Computer scientists use many abstraction levels, with reasoning and often execution tools at each level.

In 121, we learn tools for some key abstraction levels: wiring physical gates, computer-based hardware design techniques, propositional logic, predicate logic, sets, …

Functions are a new level that let us talk about how computations work and fit together.

Bonus: functions alone are enough for all of computation, using the “λ calculus”.

Outline




• Function Operations– Inverse– Composition

What is a Function?

Mostly, a function is what you learned it was all through K-12 mathematics, with strange vocabulary to make it more interesting…

A function f:A B maps values from its domain A to its co-domain B.

Domain Co-domainf(x) = x3

f(x) = x mod 4f(x) = x

Look, sets!

CORRESPONDS TO TEXTBOOK READING(NOT COVERED IN CLASS)

Plotting Functions

f(x) = x3

f(x) = x mod 4

f(x) = x

Not every function is easy to plot!


What is a Function?

Not every function has to do with numbers…


Domain Co-domainf(x) = ~x

f(x,y) = x yf(x) = x’s phone #


What is a Function?


f(control, data1, data2)= (~control data1) (control data2)

Domain?Co-domain?


What is a Function?


Domain?Co-domain? Other examples?

Alan

Steve

Paul

Patrice

Karon

George

111

121

211


What is a Function?


f can’t map one element of its domain to more than one element of its co-domain:

x A, y1,y2 B, [(f(x) = y1) (f(x) = y2)] (y1 = y2).

Why insist on this?A B

f


Not a Function

Why isn’t this a function?

(The Laffer Curve: a non-functional tax policy.)


Function Terminology


For f to be a function, it must map every element in its domain:

x A, y B, f(x) = y.

Why insist on this?

A B

f

Warning: some mathematicians would say that makes f “total”.


Not a Function

Foiled by sabbatical.

Alan

Steve

Paul

Patrice

Karon

George

111

121

211

Anne


Function Terminology


f(x) is called the image of x (under f).

x is called the pre-image of f(x) (under f).

A B

f

x

y


Trying out Terminology

f(x) = x2

What is the image of 16?What is the range of f?

x

f(x)


Functions We’ve Used

• Any combinational logic circuit

• The “inverse” and “zip” operations over algorithms from the midterm

• The operation that gives you a student’s grade given the student: G(s) = s’s grade

• Arrays, as long as they’re immutable (unchangeable): A(i) = the ith value in array A.

Sequential circuits and mutable arrays aren’t functions. Why not?(Actually we can model them as functions by making time explicit)

Outline





Function Properties: Injective

A function f:A B is injective (also one-to-one) if each image is associated with at most one pre-image: x,y A, xy f(x) f(y).

AlanStevePaul

PatriceKaron

George

111121211

AlanStevePaul

PatriceKaron

George

211/201211/202211/BCS

111121/202121/203121/BCS

Injective? Injective?


f(x) = x2

Injective?What if f:R+ R+?

x

f(x)


f:{s|s is a 121 student} {A+, A, …, D, F}f(s) = s’s mark in 121

Is f injective?

What if we didn’t know what f represented, only its “type” and the fact that there are lots of 121 students:f:{s|s is a 121 student} {A+, A, …, D, F}

Nifty Injective Function: Error-Correcting Codes

Error-correcting codes must be injective. Why?

Outline





Function Properties: Surjective

f:A B is surjective (also onto) if every element of the co-domain has a pre-image: y B, x A, y = f(x).

AlanStevePaul

PatriceKaron

George

121211

AlanStevePaul

PatriceKaron

George

211/201211/202211/BCS

111121/202121/203121/BCS

Surjective? Surjective?

Can we define “surjective” in terms of “range” and “co-domain”?


f(x) = x2

f:R R0?

Surjective?What if f:R R?What if f:Z Z0?

How about f(x) = x?For what types is it surjective?

x

f(x)


f:{s|s is a 121 student} {A+, A, …, D, F}f(s) = s’s mark in 121

Is f surjective?

Could we ever know that f was surjective just by knowing f’s domain and co-domain?

Nifty Surjective Function: “Lossy” Compression

WARNING: Lossy compression functions are not always surjective… But it’s valuable if they are. Why?

Surjective Functions So Far

Under what circumstances is a combinational circuit with one ouput surjective?

Under what circumstances is it not surjective?

So, are our circuits usually surjective or not?

Outline





Function Properties: Bijective

A function f:A B is bijective (also one-to-one correspondence) if it is both one-to-one and onto.

Every element in the domain has exactly one unique image. Every element in the co-domain has exactly one unique pre-image.

Function Properties: Bijective

A function f:A B is bijective (also one-to-one correspondence) if it is both one-to-one and onto.

AlanStevePaul

PatriceKaron

George

121211

AlanStevePaul

PatriceKaron

George

211/201211/202211/BCS

121/202121/203121/BCS

Bijective? Bijective?

111


f(x) = x2

f:? ?

Bijective for what domain/co-domain?

x

f(x)

Nifty Bijective Function: Encryption/Lossless Compression

Two sets have the same cardinality if we can put them in a bijection. What does that say about lossless compression?

Outline





Function Operations: Inverse

The inverse of a function f:A B is f-1:B A. f(x) = y f-1(y) = x.

How can we tell whether f-1 is a function?

(Hint: what would make f-1 not a function?)

Can we prove it?


What’s the inverse of each of these fs?

AlanStevePaul

PatriceKaron

George

121211

AlanStevePaul

PatriceKaron

George

211/201211/202211/BCS

121/202121/203121/BCS

111


f(x) = x2

What’s the inverse of f?

What should the domain/co-domain be?

x

f(x)

Proving a function injective

Recall: A function f : A B is injective exactly when: x,y A, xy f(x) f(y).

A typical approach is to prove the contrapositive by antecedent assumption: assume f(x) = f(y) and show that x = y.

Problem: x3 + 5 is injective

Theorem: f(x) = x3 + 5 is injective, where f : Z Z.

Proving a function surjective

Recall: A function f : A B is surjective exactly when: y B, x A, y = f(x).

That existential gives us a lot of freedom to pick a witness!

A typical approach is to “solve for” the necessary x given a y.

Problem: w + 2z is surjective

Theorem: f(w,z) = w + 2z is surjective, where f : (Z Z) Z.

Proving a function bijective

Prove that it’s injective.

Prove that it’s surjective.

Done.

Proving a function has an inverse

Prove that it’s bijective.

Done.

An Inverse Proof

Theorem: If f:A B is bijective, then f-1 : B A is a function.

Recall that f-1(f(x)) = x.

Aside: Functions are Just Sets

We can define functions in terms of sets and tuples (and we can define tuples in terms of sets!).

The function f:A B is the set {(x, f(x)) | x A}.

f is a subset of A B!

Learning Goals: In-Class

By the end of this unit, you should be able to:– Define the terms injective (one-to-one),

surjective (onto), bijective (one-to-one correspondence), and inverse.

– Determine whether a given function is injective, surjective, and/or bijective.

– Apply your proof skills to proofs about the properties (e.g., injectiveness, surjectiveness, bijectiveness, and function-ness) of functions and their inverses.

Documents

Snick snack CPSC 121: Models of Computation 2008/9 Winter Term 2 Functions Steve Wolfman, based on notes by Patrice Belleville and others