# Algebraic Structures

(代数系)

## Discrete Mathematics I

### 11th lecture, December 14, 2018

http://www.sw.it.aoyama.ac.jp/2018/Math1/lecture11.html

# Today's Schedule

• Remaining schedule, final exam
• Summary, leftovers, and homework from last lecture
• Algebraic Structures
• Groups
• Group axioms
• Examples of groups
• Permutations and symmetric groups
• Simple group theorems
• Group isomorphisms
• Cayley tables

# Remaining Schedule

• December 14: this lecture
• December 21: 12th lecture
• January 11: 13th lecture
• January 15 (Friday lectures on a Tuesday): 14th lecture
• January 18: 15th lecture (makeup class)
• January 25, 11:10-12:35: Term final exam

About makeup classes: The material in the makeup class is part of the final exam. If you have another makeup class at the same time, please inform the teacher as soon as possible.

# Final Exam・期末試験

Coverage:
Complete contents of lecture and handouts
Past exams: 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017
How to view example solutions:
Press [表示 (S)] button or [S] key. To revert, press [非表示 (P)] button or press [P] key.
Sometimes, more than one key press is needed to start switching.
Some images and example solutions are missing.
Important points:
Read problems carefully (distinguish between calculation, proof, explanation,...)
Be able to explain concepts in your own words
Be able to do calculations (base conversions, truth tables,...) speedily
Combine and apply knowledge from different lectures
Write clearly

# Leftovers of Last Lecture

Hasse diagrams, equivalence relations and order relations in matrix representation

# Summary of Last Lecture

We defined the following properties of binary relations:

1. Reflexive: xA:xRx; ∀xA: (x, x) ∈ R
2. Symmetric: ∀x, yA: xRyyRx;
x, yA: (x, y) ∈ R ⇔ (y, x) ∈ R
3. Antisymmetric: ∀x, yA: xRyyRxx=y
4. Transitive: ∀x, y, zA: xRyyRzxRz

A relation that is reflexive, antisymmetric, and transitive is a (partial) order relation.

A relation that is reflexive, symmetric, and transitive is an equivalence relation.

(Partial) order relations can be represented with Hasse diagrams.

# Last Week's Homework: Combinations of Properties of Relations

Sorry, it was removed! :)

# Algebraic Structure

Very general view on mathematical objects

An algebraic structure is a class of mathematical objects that all share the same general structure.

Properties shared by all algebraic structures are:

• A set (or more than one set)
• An operation on the elements of the set
(more than one operation in some cases)
Condition: The results of the operation(s) also have to be elements of the set
This is called closure; the set is closed under the operation
• Some axioms
• Proofs of theorems and properties from the axioms

# Previously Encountered Examples

• Plane geometry and the Euclidean axioms
• Natural numbers and the Peano axioms
• Truth values (true/false) and the axioms of basic logic (several choices)
• Up to now:
• Try to axiomatize
• Today:
• Try to find a small set of "interesting", "productive" axioms
• Look at commonalities among different sets and different operations

# Example of Algebraic Structure: Group

• One set (A)
• One binary operation (•; the set is closed under the operation: ∀b,cA: bcA)
• Three axioms:
1. Associativity (∀b,c,dA: (bc)•d = b•(cd))
2. (Existence of a) identity element e (∃eA: ∀bA: eb = b = be)
3. (Existence of an) inverse element b' (∀bA: ∃b'∈A: bb' = e = b'•b)
(The inverse element may also be written b-1)
• Note: Commutativity is not necessary

# The Integers with Addition as a Group (ℤ, +)

• Set: ℤ (integers)
• Associativity: ∀b,c,d∈ℤ: (b+c)+d = b+(c+d)
• Identity element: 0
• Inverse element: b' = -b

# The Reals with Multiplication as a Group (ℝ-{0}, ·)

• Set: ℝ-{0} (real numbers without 0)
• Operation: · (multiplication)
• Associativity: ∀b,c,d∈(ℝ-{0}): (b·cd = b·(c·d)
• Identity element: 1
• Inverse element: b' = 1/b (inverse/reciprocal, b-1)

# The Positive Reals with Multiplication as a Group (ℝ+, ·)

• Set: ℝ+ (positive real numbers)
• Operation: · (multiplication)
• Associativity: ∀b,c,d∈ℝ+: (b·cd = b·(c·d)
• Unit element: 1
• Inverse element: b' = 1/b

# Permutations

• There are n! permutations of elements from a set S with size |S|=n
• Permutations can be seen as ordered selections
Example: From the set {Aoyama, Sagamihara} we can create the permutations (Aoyama, Sagamihara) and (Sagamihara, Aoyama)
Example: From the set {cat, dog, horse, cow}, we can select the permutation (dog, cow, cat, horse) (and 23 others)

# Permutations as Exchanges

• Permutations can be seen as ways to exchange elements
Example: For a tuple/list with two elements, there are two permutations:
1. One permutation that keeps the same order: (1, 2)
2. One permutation that changes the order of the elements: (2, 1)
• We denote such permutations by assuming we start with a tuple of the first n integers ((1, 2,...)), and show the result of the permutation
• Example: The tuple (cat, dog, horse, cow), when permuted with the permutation (2, 4, 1, 3), results in (dog, cow, cat, horse)

# Composition of Permutations

• When seen as exchanging elements, permutations can be composed
• We use ∘ to denote composition
• Example: (2, 4, 1, 3) ∘ (1, 4, 2, 3) = (2, 3, 4, 1)
• Composition of permutations can be show by using cards
• Cut out and use the cards at permutations.svg

# Symmetric Groups

• The permutations of sets of size n together with composition form a group:
• All compositions of permutations result in another permutation
• Permutations are associative
• The identity element is (1, 2, 3, 4, ...)
• Each permutation has an inverse
Example: The inverse of (2, 4, 1, 3) is (3, 1, 4, 2)
• Commutativity does not hold
Example: (2, 4, 1, 3) ∘ (1, 4, 2, 3) = (2, 3, 4, 1)
(1, 4, 2, 3) ∘ (2, 4, 1, 3) = (4, 3, 1, 2)
• These groups are called symmetric groups of order n

# Group Theorem: Uniqueness of Identity

Existence of identity element: ∃eA: ∀bA: eb = b = be

Theorem: The identity element of a group is unique
(∃cA: ∀xA: cx = x) ⇒ c = e

Another way to express this: There is only one identity element
(|{c|cA, ∀xA: cx = x)}|= 1)

Proof:

cx = x [inverse axiom, closure]

(cx)•x' = xx' [associativity axiom]

c•(xx') = xx' [inverse axiom, on both sides]

ce = e [identity axiom]

c = e Q.E.D. (similar proof for right idenity)

# Group Theorem: Uniqueness of Inverse

Existence of an inverse: ∀bA: ∃b'A: bb' = e = b'•b

Theorem: Each inverse is unique
a, b∈A: (ab = eb=a')

Proof:

ab = e [applying a'• on the left]

a'•(ab) = a'•e [associativity axiom]

(a'•a)•b = a'•e [inverse axiom]

eb = a'•e [identity axiom, on both sides]

b = a' Q.E.D. (similar proof for left inverse)

# Group Theorem: Cancellation Law

Theorem: ∀a, b, c ∈A: (ac = bca=b)

Proof:

ac = bc [applying c' on the right]

(ac)•c' = (bc)•c' [associativity]

a•(cc') = b•(cc') [inverse axiom, on both sides]

ae = be [identity axiom, on both sides]

a = b Q.E.D. (similar proof for left cancellation)

# Group Isomorphism

• Two groups (G, •) and (H, ∘) are isomorphic if there is a function f so that:
• hH: ∃gG: h = f(g)
• g1, g2G:g1g2f(g1)≠f(g2)
• g1, g2G: f(g1g2) = f(g1)∘f(g2)

The elements in G and H correspond one-to-one, and the operation works exactly the same on corresponding elements.

• If two groups are isomorphic
• They have the same number of elements (|G|=|H|)
• They have the same structure
• From a mathematical viewpoint, they can be considered to be the same

# Examples of Isomorphic Groups

• Example 1: (ℝ, +) is isomorphic to (ℝ+, ·), with f(x) = ax (a>1)
• Example 2: Three isomorphic groups (shown as Cayley tables)
e a b G e a b a b e b e a
0 2 1 K 0 2 1 2 1 0 1 0 2
0 1 2 H 0 1 2 1 2 0 2 0 1

# Cayley Tables

• Finite groups are usually described using Cayley tables
• Cayley tables look very much like multiplication tables
• Conventions:
• The left operands are used as the row headings
• The right operands are used as the column headings
• The set and/or the operation is placed in the upper left corner
• The identity element is placed in the first (actual) row and column
• Properties:
• The first row/column is the same as the headings (reason: identity element)
• Each element of the set appears once in each row/column (reason: cancellation law)
• The identity element is distributed symmetrically to the diagonal (reason: inverse element)
• Associativity has to be checked "by hand"

# This Week's Homework

Deadline: December 20, 2017 (Thursday), 19:00.

Format: A4 single page (using both sides is okay; NO cover page), easily readable handwriting (NO printouts), name (kanji and kana) and student number at the top right

Where to submit: Box in front of room O-529 (building O, 5th floor)

Homework 1: Create a Cayley table of the symmetric group of order 3. Use lexical order for the permutations. Use the notation introduced in this lecture.

Homework 2: If we define isomorphic groups as being "the same", there are two different groups of size 4. Give an example of each group as a Cayley table. Hint: Check all the conditions (axioms) for a group. There will be a deduction if you use the same elements of the group as another student.

# Glossary

algebraic structure

group

group theory

inverse element

inverse, reciprocal

symmetric group

closure

Cayley table

multiplication table

isomorphic

group isomorphism