Tour:Group

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This article adapts material from the main article: group

This page is part of the Groupprops guided tour for beginners (Jump to beginning of tour)
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Expected time for this page: 15 minutes
General instructions for the tour | Pedagogical notes for the tour | Pedagogical notes for this part
WHAT YOU NEED TO DO:
  • Read, and understand thoroughly, the definition of group given below.
  • Go through the information on notation and conventions given here, since these conventions will be used throughout the wiki

IF YOU ARE FOLLOWING ANOTHER PRIMARY TEXT:

  • Compare the definition of group given here with the definition in that text, and make sure the definitions match up.
  • Compare the notations and conventions introduced here with those given in that text, and make sure the notations match up.


Definition

QUICK PHRASES: monoid with inverses, set with associative binary operation having identity element and inverses, symmetries of a structure
Understand the definition better at understanding the definition of a group: clarify your doubts
Learn how to apply this definition to verify the group axioms in concrete situations

The textbook definition (with symbols)

A group is a set G with a binary operation *: G \times G \to G (termed the multiplication or product and denoted with infix notation -- so the product of x,y is denoted x * y) such that the following hold:

Condition name What it says Comments
associativity For any a,b,c in G, (a * b) * c = a * (b * c). Note that a,b,c are allowed to be equal (i.e., there is no restriction on whether they are equal or not). For instance, all may be equal, all distinct, or two equal and one different.
identity element (or neutral element) There exists an element e in G such that a * e = e * a = a for all a in G. Such an e is termed an identity element or neutral element for G. Note that what this is saying is that a single choice of e works for all a. It turns out, from binary operation on magma determines neutral element, that the identity element of a group is unique, so we can call it the identity element, but this is not a priori obvious.
inverse elements (this condition depends on the identity element condition) For any a in G, there is an element b such that a * b = b * a = e, where e is an identity element chosen for the previous condition. Such a b is termed an inverse of a and is denoted as a^{-1}. It turns out, from two-sided inverse is unique if it exists in monoid, that the inverse of an element in a group is unique, so we can call it the inverse element, but this is not a priori obvious.

The data describing a group include both the set and the binary operation. In other words, if we're just given a set, it doesn't make sense to ask whether it's a group. If we're given a set with a binary operation, it makes sense to ask whether the set and the binary operation together define a group structure. In particular, different binary operations on the same set could define different group structures.

The group can therefore be explicitly described as the pair (G,*).

Note on closure axiom: Some texts include a closure axiom in the group definition, which says that for all x,y \in G, x * y \in G. This condition is not explicitly stated here because it is part of the definition of binary operation, and checking closure is part of checking that the binary operation is well-defined.

The video below includes both the textbook and the universal algebra definitions.

The universal algebra definition (with symbols)

A group is a set G equipped with three operations:

Operation name Arity of operation Operation description and notation
Multiplication or product 2 A binary operation *: G \times G \to G (infix operator) termed the multiplication or product. The product of x and y is denoted x * y.
Identity element (or neutral element) 0 A 0-ary operation which gives a constant element, denoted by e (sometimes also as 1), termed the identity element or neutral element.
Inverse map 1 A unary operation {}^{-1}:G \to G (superscript operator) termed the inverse map. The inverse of x is denoted x^{-1}.

satisfying the following three compatibility conditions:

Condition name Minimum number of variables to describe condition Condition description Comments
Associativity 3 For all a,b,c in G, we have (a * b) * c = a * (b * c). Note that a,b,c are allowed to be equal (i.e., there is no restriction on whether they are equal or not).
Identity element (or neutral element) 1 For all a in G, we have  a * e = e * a = a Note that, unlike the textbook definition, the identity element e is specified as part of the group structure using a 0-ary operation. Thus, we do not start with the "there exists an element e \in G" clause.
Inverse element 1 For all a in G, we have a * a^{-1} = a^{-1} * a = e Not that, unlike the textbook definition, the inverse map is specified as part of the group structure as a unary operation. Thus, we directly write the condition in terms of that operation instead of writing "there exists b \in G such that ..."

The entire collection of information describing a group is sometimes written as a 4-tuple: (G,*,e,{}^{-1}).


Equivalence of definitions

For full proof, refer: Equivalence of definitions of group

The textbook definition includes only the set and the binary operation in the group structure, whereas the universal algebra definition additionally includes the identity element and the inverse operation in the group structure. To show that these definitions are equivalent, we need to demonstrate that the binary operation on the set uniquely determines the identity element and the inverse map. There are two parts to the proof:

Further term: abelian

For a group, * may not be commutative, viz., it may not be true that a * b = b * a for all a and b in the group. If the group satisfies the additional property that a * b = b * a for all a,b in the group, it is termed abelian.

Notation

Notation for group operations and expressions

The binary operation of the group is often called multiplication and its application is termed product. Because it is associative, we can drop the operator symbol as well as parenthesization (refer associative binary operation#Parenthesization can be dropped).

The inverse map is denoted by a superscript postfix (applied to g, it looks like g^{-1}).

The identity element is denoted by e, or sometimes, by 1.

Notations are somewhat different for an abelian group.

Here is a summary of important things to remember:

  1. Because the group operation is associative, we often drop both the bracketing and the group multiplication symbols while writing products of elements in the group. Thus:
    • (a * (b * d)) * c is written as abdc
    • a * (c * g)^{-1} is written as a(cg)^{-1}
  2. The identity element is often denoted as 1 or e.
  3. Repeated multiplication map is denoted by powers. So x * x is x^2 while x^{-1} * x^{-1} is x^{-2}. Similarly, x * (x * x) = xxx is written in short as x^3.
  4. The inverse superscript binds only to the immediately preceding variable or parenthesized expression. So xy^{-1} means x * (y^{-1}) rather than (x * y)^{-1}.
  5. Power superscripts, like the inverse superscript, also bind to the immediately preceding variable. For instance, xy^2 means x * (y^2) rather than (x * y)^2.

Notation for the group and its set-theoretic constructions

Groups are typically denoted by capital English or Greek letters such as G,H,K or \Gamma, \Lambda. Usually a group is confused with its underlying set, so we can talk of subset of a group. It must be remembered, however, that meaning is associated to the set only with the extra structure of the group operations.

Elements of the group are denoted by small letters (such as g, h). The identity element is denoted as e or 1. (For abelian groups, the identity element is denoted by 0).

Subsets of the group are again denoted by capital letters, and subset inclusions are denoted by \subseteq. When talking of subgroups, we typically use \le to emphasize that the subset also has a group structure.

Complete descriptions of groups

  • To describe a group with the textbook definition, we need to provide both the underlying set and the binary operation. To emphasize this, we write the group as a tuple of the set and the binary operation. For instance, we write (G,*) to denote the group G with binary operation *.
  • To describe a group with the universal algebra definition, we must also specify the identity element and the inverse operation as part of the group structure. For this, we write the group as a 4-tuple of the set, binary operation, identity element and inverse map. For instance: (G,*,e,{}^{-1}) denotes the group G with binary operation *, identity element e and inverse map {}^{-1}.

Examples

Occurrence of groups

Further information: Occurrence of groups

Groups occur in many avatars. Examples of abelian groups include the additive groups of real numbers, of rational numbers, of complex numbers, and of integers, and the multiplicative groups of nonzero real numbers, of nonzero rational numbers, of nonzero complex numbers. In particular:

Tuple description of group Underlying set Binary operation (group multiplication) Identity element Inverse map Comment
(\R^*,\times,1,{}^{-1}) The nonzero reals \R^* Multiplication of (nonzero) real numbers 1 The usual multiplicative inverse or reciprocal. For instance, the inverse of \sqrt{2} is 1/\sqrt{2} = \sqrt{2}/2
(\mathbb{Z}, +, 0, -) The integers \mathbb{Z} Addition of integers 0 The negative. For instance, the additive inverse of -2 is 2. See group of integers for more information.
(\mathbb{Q}^*, \times, 1, {}^{-1}) The nonzero rational numbers \mathbb{Q}^* Multiplication of (nonzero) rational numbers 1 The usual multiplicative inverse or reciprocal. For instance, the inverse of -2/7 is -7/2.

On the other hand, the following are not groups:

  1. The nonnegative integers under addition: There is an identity element, namely 0. However, the additive inverse of a nonnegative integer is not always a nonnegative integer, so the set of nonnegative integers does not have additive inverses. Hence, it is not a group.
  2. The nonzero integers under multiplication: There is an identity element: 1. However, not every integer has a multiplicative inverse, so this set does not have multiplicative inverses. Hence, it is not a group.
  3. The set of all rational numbers under multiplication: There is an identity element: 1. However, the element 0 does not have a multiplicative inverse, so the rational numbers do not form a group.
PONDER (WILL BE EXPLORED LATER IN THE TOUR):
  • Over the different components of definition of a group, and why the various definitions are equivalent
  • Over any examples of groups you may have already seen, and how the various components of the definition play a role in those particular examples
This page is part of the Groupprops Guided tour for beginners (Jump to beginning of tour). If you found anything difficult or unclear, make a note of it; it is likely to be resolved by the end of the tour.
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