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Tue Jul 31 13:55:51 PDT 2012

# Group Theory

## Motivation

Groups turn up whenever we have a set of reversible operations. The numbers with adddition, numbers (without zero) and multiplication, the movements of a figure on the plane, or a beer glass in space all follow a similar pattern of operations that we call a group. A group is anything that has most of the algebraic properties of the numbers with respect to either addition or multiplication (but not both). Groups are are a rich and well explored algebra.

## Basics

1. monoids::= See http://cse.csusb.edu/dick/maths/math_33_Monoids.html

2. |- (basis):

3. GROUP::=following
Net
1. Set::Sets,
2. operation::Associative(Set),
3. op::=operation,
4. unit::units(Set,op).
5. u:=unit.
6. |- (G0): MONOID(Set, op, u).
7. inverse::Set->Set,
8. i:=inverse,
9. |- (G1): for all x:Set, x op i(x)=u=i(x) op x.

(End of Net)

4. |- (Group_is_algebra): ALGEBRA(GROUP, Group, group).

5. (Group_is_algebra) |- Group::=\$ GROUP.
6. (Group_is_algebra) |- group::={Set:Sets || GROUP(Set, op, u, i)},

7. group::={Set:Sets || GROUP(Set, operation=>(1st), unit=>(2nd), inverse=>(3rd))}.
8. monoid::Group->Monoid=(Set=>(_).Set, op=>(_).operation, unit=>(_).unit).
9. semigroup::Group->Semigroup=(Set=>(_).Set, op=>(_).operation).
10. congruence::Group->Equivalence_relations=congruence(monoid(_)).

## Alternative definition

11. |- (one-sided): Given just left identities and inverse then a monoid must be a group.

Source: Georges Pappy's book on Groups and Birkhoff and Bartee 70, page 213.

12. |-Group=\$ PAPPY.
13. PAPPY::=following
Net
1. A semigroup in which equations can be solved.
2. |- (PG0): SEMIGROUP(Set, op).
3. u::Set.
4. i::Set---Set.
5. |- (PG1): for all a,b:Set, some x,y :Set (a op x = b = y op a).
6. (PG1)|- (PGu): u= the x (for all a (a op x=a=y op a)).
7. (PG1, PGu)|- (PGi): i= the i( for all a ( a op i(a) =u)).

## Proof of PGu

Let

1. |-a:G.
2. (PG1(a=>a, b=>a))|-for some x,y:Set(a op x=a=y op a).
3. (above)|-some left_units and some right_units.
4. (above)|-one unit.

(Close Let )

## Proof of PGi

Let

1. |-a:G,
2. (PG1(a=>a, b=>u))|-inverses
3. ...

(Close Let )

(End of Net)

## Properties

[ unique_inverses in math_33_Monoids ]
14. (above)|- (unique unit): For all Group G, one u:G ( for all x:G( x op u = u op x = x)).
15. (above)|- (unique inverse): For all Group G, one i:G ( for all x:G( x op i(x)= u )).
16. (above)|- (cancelation): For all Group G, for all a:G ( for all x,y(if a op x = a op y then x=y) and for all x,y(if x op a = y op a then x=y).
17. (above)|- (equation_solvable): For all G,a,b( for one x( a op x =b) and one x ( x op a =b ) ) .
18. (above)|- (solve_equation): For all G,a,b( the x( a op x =b)= i(a) op b and the x ( x op a =b )=b op i(a) ) .
19. (above)|- (inv unit): For G, i(u)=u.
20. (above)|- (dist inv): For G, i(a op b) = i(b) op i(a).

Note, that by the STANDARD defitions (op) is a serial operator, [ math_11_STANDARD.html ]

21. (above)|- (sum): For G, A,B:Finite_Sets, f:G^(A|B), op(f) = op(f o B) op op(f o B) op i( op(f o (A&B))).

## Special groups

22. m_group::=multiplicative_group.
23. multiplicative_group::={Set||GROUP(op=>., u=>1, i=>map s(s^-1)) and Net{ For x,y:Set, (x y):=x.y}}.

25. additive_group::={Set||GROUP(op=>+, u=>0, i=>-) and Net{+ in commutative(Set)}}.

26. Abelian_group::=\$ GROUP and Net{ op in commutative(Set)}
27. Traditionally commutative groups use additive notation while non_commutative groups use the multiplicative notation:

Do properties like (ab)^n=a^n b^n force a group to be abelian? [ Gallian & Reid 93, "Abelian Forcing Sets", Am Math Monthly June-July 93 pp580-582] .

28. FREE_GROUP::=
Net{
1. Elements::Finite_sets.
2. Inverse::Elements --- Elements.
3. |-GROUP( Set=> #( Elements | Inverses), op=>(!), u=>(), i=>Inverse).
4. |-For all s1,s2:Set, e:Elements, s1 ! e ! Inverse(e) ! s2 = s1! s2.

}=::FREE_GROUP.

29. For S:Finite_sets, i:S---S, Free_group(S, i)={Set || FREE_GROUP( Elements=>S, Inverse=>i, ...)}.

30. (above)|-Free_group(S,i)=Monoid( S || for all s( s i(s) = i(s) s = u ) ).

## Multiplicative Groups

### Congruency

1. For G:m_group~numeric, x,y:G, x^y::=y^-1.x.y
2. For groups that contain numbers x^n is ambiguous - use x^^y for congruency

### Commutator

3. For G:m_group, x,y:G, [x,y]::=x^-1.y^-1.x.y

. . . . . . . . . ( end of section Multiplicative Groups) <<Contents | End>>

## Actions of a Group

The isomorphisms of a set form a group.
31. For set S, iso(S)::group= GROUP( S---S, (o), Id(S), (map[i](/i)) ).

As in monoid theory, we can define the actions of a group -- but the actions must be reversible:

32. For M:group, S:set, actions(M,S)::=(group)(M)->iso(S).

Most of the results [ math_33_Monoids.html#Actions of a Monoid ] for monoids can be applied to Actions of a Group as well. We can also show some new results like the Burnside Lemma

Here is a more traditional presentation of actions of a group action

33. GROUP_ACTION::=following
Net
1. G:: group(o, u, i) = given,
2. X:: Sets = given,
3. *::G >< X -> X.
4. |- (GA1): for all x, u * x = x
5. |- (GA2): for all x, g1,g2, (g1 o g2) * x = g1 * ( g2 * x).

We say that X is a G-Set.

G[x] ::= {g || g*x = x},

X[g] ::= {x || g*x = x},

6. x1 ~ x2::= for some g, g*x1=x2.
7. G x::= x/~.
8. (above)|-for x, G x sub_group G.

9. (above)|-|G x| = |G : G[x]| = |G| / |G[x]|.

10. orbit = {G x || for some x}.

11. (above)|-(burnside_s_lemma) : |orbit| * |G| = +[g:G] (X[g]).

(End of Net GROUP_ACTION)

## Subgroups

34. For G:\$ GROUP, subgroups(G)::={ H:@G||H in \$ GROUP},
35. For H,G, H sub_group G::= H in subgroups(G).
36. (above)|-for H:@G(H sub_group G iff for all x,y:H(x op i(x) in H).
37. (above)|-{{u},G}==>sub_groups(G).
38. (above)|-(Set=>sub_groups(G),R=>==>)in poset.
39. For A:@G, <A>::=min{H:subgroups(G)||A==>H}.
40. (above)|-<A>={op(x)||x:#(A|i(A))} =( H:@G||for all a:A,h:H({h^-1, a op h, h op a}==>H)).

41. normal_subgroups(G)::={H:subgroups(G)||for all x:G(x.H=H.x)}.
42. normal(G)::=normal_subgroups(G).

43. For X,Y:subgroups(G), X orthogonal Y::@=( X&Y={u} and X op Y =G).

## Generators and Relators

44. For S:@G, generated_group(S)::=the smallest { H : subgroups(G) || S==> H }. .Example
45. DIHEDRAL::=Net{n:Nat. n>2. m_group(#{R, V}). V V=1. R^n=1. R V = V R^-1.}

46. For A:Finite_set, i: A---A, S:@#A , Group(A||S)::=Free_group(A, i )/R
47. where R=min{E:congruence(#A)||for all s:S(s E 1)} So the Dihedral group G in \$ DIHEDRAL is isomorphic to Group({R,V}|| VV, V^-1 R V R^-1, R^n).

## Morphisms

48. For G1,G2:\$ GROUP, a:arrow, (group)G1 a G2::=(monoid) Monoid(G1) a Monoid(G2)& {f || f o i=i o f}.

Source: Birkhoff & Bartee 70 (above) |-For G1,G2:\$ GROUP, a:arrow, (group)G1 a G2 ::= (semigroup) Semigroup(G1) a Semigroup(G2). .Note However a semigroup morphism between monoids may not be a monoid morphism.

## Left Cosets.

49. For G:m_group, H: sub_groups(G), x,y:G, (x = y wrt H) ::=(x.H=y.H),

(=wrt H) ::=rel [x,y](x=y wrt H),

for x:G, x/H::=x/(=wrt H),

50. left_cosets(H,G)::=G/H.

51. |- (LC0): For G,H, (=wrt H) in congruence(monoid(G)).

52. COSETS::=
Net{
1. G::m_group,
2. H::sub_groups(G).
3. (def)|- (C1): x/H={y || y.H=x.H},
4. (def)|- (C2): left_cosets(H,G)= G/(=wrt H).
5. (def)|- (C3): For x,y,H, x=y wrt H iff y^-1.x in H
6. (above)|-(C4) for all x:G (x.H=x/H).

## Proof of C4

7. Let{ x:G,
8. (C4.0): x/H={y||y.H=x.H},
9. (above)|- (C4.1): ( for all h,some g(y=x g h^-1) and for all h, some g(y=x h g^-1) iff for some h(y=x h).
10. LHS::= for all h,some g(y=x g h^-1)&for all h, some g(y=x h g^-1),
11. RHS::=for some h(y=x h).
12. (above)|- (C4.2): if LHS then RHS.
13. (above)|- (C4.3): if RHS then LHS.
14. x/H= {y||for some h(y=xh)}
15. }.

## Proof of C4.2

16. Let{
17. |- (C4.2.0): LHS,
18. |- (C4.2.1): not RHS.
19. (C4.2.1)|- (C4.2.2): for all h(y<>x h).
20. (C4.2.0)|- (C4.2.3): for all h, some g(y=x g h^-1).
21. (C4.2.0)|- (C4.2.4): for all h, some g(y=x h g^-1).
22. (C4.2.3, h:=h0, g:=g0)|- (C4.2.5): y=x g0 h0^-1 .
23. (C4.2.2, h:=g0 h0^-1)|- (C4.2.6): y<>x g0 h0^-1 .
24. (C4.2.5, C4.2.6)|-y <> y.
25. }.

## Proof of C4.3

26. Let{
27. |- (C4.3.0): not LHS,
28. |- (C4.3.1): RHS,
29. (above)|- (C4.3.2): y=x h0.
30. (above)|- (C4.3.3): for some h,all g(y<>x g h^-1) or for some h, all g(y<>h g^-1)
31. (above)|- (C4.3.4): all g(y<>x g h1^-1) or all g(y<>x g h1^-1)
32. (above)|- (C4.3.5): either all g(y<>x h1 g^-1) or all g(y<>x h1 g^-1)
33. (above)|- (C4.3.6): y<>x h0 h1 h1^-1
34. (above)|- (C4.3.7): g=h1 h0
35. }.

36. (above)|- (C4.4): for all x:G(x.H in G/H).

37. (above)|- (C4.5): for all x:G (A;(x._) in A---x.A).

38. (above)|- (C4.6): for all x:G, (x.A);(x^-1._) = / (x._).

39. (above)|- (C4.7): for all A:G/H( A---H ).

40. (above)|- (C4.8): If H in normal(G), map[x](x/H) in (group)G>==G/H

}=::COSETS.

53. (above)|-For all G,H(COSETS).

## Kernels Cosets and morphisms

As with monoids the possible morphisms from a group into another group are reflected in the normal subgroups. For any morphism f:(group)G1>->G2 we can define a normal subgroup of G1 called the kernel of f:
54. ker(f) ==> G1 >==coi(f) --- img(f) ==> G2.

Given a normal subgroup N of G1 then we can define a group morphism G1>==G1/N.

## Burnside's Theorem

55. (above)|- (burnside): For G:m_group, H:normal(G), |G| = |G/H| * |H|.

56. (above)|-For G1,G2:group, f:(group)G1->G2, ker(f) in normal(G1) and G1/ker(f)=coi(f)

57. (above)|-for all G1,G2:group, f:(group)G1>--G2, |G1|=|G2|*|ker(f)|
58. (above)|-For H,K:normal(G), if H==>K then G/H>--G/K
59. |-For G,X:group, x:(group)G->X, some Y:group, y:(group)G->Y ( G---\$ Net{x:X y:Y}and ker(x)orthogonal ker(y) ).

## Direct Sum

60. (above)|-For H,K:sub_group(G), G---H><K iff H orthogonal K and for all h:H,k:K(h.k=k.h)

## Abelian Direct sums

61. (above)|-For G:a_group, A,B:sub_group(G), G---A><B iff A orthogonal B.

## Direct products of Groups

62. For G:m_group, p:1.., G^p=G^(1..p).
63. For G:m_group, S:Set, G^S in m_group and
64. for all f,g:G^S
65. f.g=map x(f(x).g(x))
66. and f^-1=map x((f(x))^-1)
67. and 1= map x(1).

68. For G1,G2:m_group, S:Set, h:(group)G1->G2, (h o (_)) in G1^S(group)->G2^S.
69. For G1,G2:group, G1><G2::= the GROUP( Set=> G1.Set><G2.set, op=> map x,y:Set((x(1) G1.op y(1),x(2) G2.op y(2))), u=> (G1.u, G2.u), i=> map x((G1.i(x(1)),G2.i(x(2)))) ).

## Bilinear Maps in Abelian Groups

70. For A,B,C:a_group, (bilinear) A><B->C::={f^C(A><B) || for all a:A( (f(a,(_)))in (group)B->C) and for all b:B((f((_),b)) in (group) A->C).

## Groups of permutations of a set

71. For S:Set, permutations(S)::={G:operators(S)||for g in G(g:S---S)}

72. For A are G, T are S, A.T={g.s||g in A and s in T}, G/s::={g || g.s=s}, S/g::={s || g.s=s},
73. Orbit(T)::={G.{s} || s in T},
74. Isotropy(G,s)::={/s || s in S, invariant(T)(G) ::={T/g || g in G},
75. s1=s2 wrt G::= for some g(x=g y). s1(= mod G)s2::=s1=s2 wrt G.

76. (above)|- (GP0): for all s(G/s in m_group).
77. (above)|- (GP1): for all s(G/s(group)==>G).
78. (above)|- (GP2): (=mod G) in equivalence_relation
79. (burnside)|- (GP3): |S/(=mod G)| = +[g:G](|S/g|)/|G|
80. Average orbit

81. For S:Set, G:permutations(S), base(S,G)::={B:@S|| G.B=S and no (G~{1}).B &B}.

82. for all s:S, one (b,g):B><G(s=g.b),
83. ?? for all B'=>>B, B' not_in base, ?
84. ?? for all B'<<=B, B' not_in base(s,G), ?
85. for all B1,B2:Base(S,G)( B1---B2), S---B><G.

86. (above)|-For S:Set, G:permutations(S), s:S, G/s={g || g.s=s}, some (group)G/S->G
87. For S:Set, G:permutations(S), s:S, x,y:G, x=y wrt s::= x.s=y.s.

88. |-For S:Set, G:permutations(S), s:S, x,y:G, (
1. x=y wrt s in equivalence_relation(G)
2. and x=y wrt G/S iff x=y wrt s
) [click here if you can fill this hole]

## Permutation Groups and the Symmetric Groups

1. For n:Nat, Symmetric_group(n)::=(Set=>(1..n---1..n), op=>o, u=>I, i=>(/)).
2. For n:Nat, S(n)::=Symmetric_group(n).
3. Use multiplicative notation

### Cycle notation

A permutation of a finite set can be described as a product of cycles. A Cycle is a list of elements where the permuation changes each element into the next one. For example cycle(1,2,3) maps 1 to 2, 2 to 3, and 3 to 1.

4. For Nat m<n, l:1..m-->1..n, cycle(l)::= the p:S(n) ( for i:1..m-1(p(l(i))=l(i+1)) and p(l(m))=l(1) and for s:S(n)~img (l)(p(s)=s)).

Cycles are independent if they have no common elements:

5. no(img l1 & img l2).

Independent cycles commute.

[click here if you can fill this hole]

for f:Nat, f_cycle(p)::={q || for some m:Nat0(q=f^n(p))},

6. f_period(p)=min{m:Nat0||f^m(p)=p}

### Cannonical decomposition

The order of a set of disjoint cycles does not matter and there is a cannonical order that can be defined.
7. For f:S[n], f=*[i:1..l](cycle(l(i))) and for all i<j ( no (img(l[i])&img(l[j] and l[i](1)=l[j](1)) and for all i(l[i](1)=min img l[i])

### Congruent permutations

8. For G sub_group S[n] for s,t:G, s--t ::= for some g:G(s=g t g^-1)

### Type of a permutation in S[n]

9. type(f)::=map i(|l[i]| where l=the cannonical cyclic decomposition)
10. |-s--t iff type(s)=type(t)

### Orbits

11. For G subgroup S[n], X=1..n, (x=y wrt G) ::= some g in G(x=g(y)),
12. orbit(G)::=X/=wrt G, fix[G](K) ::= {g:G||g[k]=g}, Orb[G](K) ::=k/=wrt G.

13. (above)|-For all K, |G|= |fix[G](K)| . |Orb[G](K)|, #orbit(G)=+[g:G]|a/g|/|G|

### Laplace's Theorem

14. (above)|- (laplace): for all G:Finite&Group, some n:Nat0((group)G-->S[n])

### Proof of laplace

15. Let{

1. |-n=|G|,
2. (above)|-some c: G---1..n.(c:G---1..n)
3. (above)|-c in G---1..n and /c in 1..n---G and c;/c=Id(G) and /c;c=Id(1..n).
4. e:=map [x:G](/c;(x._);c).
5. (above)|- (1): for each a, e(x) in S[n],
6. (above)|- (2): e(1)=S[n].unit.
7. (above)|- (3): e(a.b)=e(a) e(b).
8. (1, 2, 3)|- (4): e in (group)G->S[n]
9. (above)|- (5): for a<>b, e(a)<> e(b).
10. (5)|-e in (group)G-->S[n]
}

. . . . . . . . . ( end of section Permutation Groups and the Symmetric Groups) <<Contents | End>>

## Geometries, pattern and symmetry

Studying the symmetry groups of objects in space is a way of studying space itself.

It is also put to use in chemistry and physics wherein the categorisation of all possible symetries in 3 and 4 dimensional space has some significant implications. M C Escher's research into the symetry's of 2 dimensional space is the basis of much of his art.

## Supplements and factors

[click here if you can fill this hole]

. . . . . . . . . ( end of section Group Theory) <<Contents | End>>

# Notes on MATHS Notation

Special characters are defined in [ intro_characters.html ] that also outlines the syntax of expressions and a document.

Proofs follow a natural deduction style that start with assumptions ("Let") and continue to a consequence ("Close Let") and then discard the assumptions and deduce a conclusion. Look here [ Block Structure in logic_25_Proofs ] for more on the structure and rules.

The notation also allows you to create a new network of variables and constraints. A "Net" has a number of variables (including none) and a number of properties (including none) that connect variables. You can give them a name and then reuse them. The schema, formal system, or an elementary piece of documentation starts with "Net" and finishes "End of Net". For more, see [ notn_13_Docn_Syntax.html ] for these ways of defining and reusing pieces of logic and algebra in your documents. A quick example: a circle might be described by Net{radius:Positive Real, center:Point, area:=π*radius^2, ...}.

For a complete listing of pages in this part of my site by topic see [ home.html ]

# Notes on the Underlying Logic of MATHS

The notation used here is a formal language with syntax and a semantics described using traditional formal logic [ logic_0_Intro.html ] plus sets, functions, relations, and other mathematical extensions.

For a more rigorous description of the standard notations see

1. STANDARD::= See http://www.csci.csusb.edu/dick/maths/math_11_STANDARD.html

# Glossary

2. above::reason="I'm too lazy to work out which of the above statements I need here", often the last 3 or 4 statements. The previous and previous but one statments are shown as (-1) and (-2).
3. given::reason="I've been told that...", used to describe a problem.
4. given::variable="I'll be given a value or object like this...", used to describe a problem.
5. goal::theorem="The result I'm trying to prove right now".
6. goal::variable="The value or object I'm trying to find or construct".
7. let::reason="For the sake of argument let...", introduces a temporary hypothesis that survives until the end of the surrounding "Let...Close.Let" block or Case.
8. hyp::reason="I assumed this in my last Let/Case/Po/...".
9. QED::conclusion="Quite Easily Done" or "Quod Erat Demonstrandum", indicates that you have proved what you wanted to prove.
10. QEF::conclusion="Quite Easily Faked", -- indicate that you have proved that the object you constructed fitted the goal you were given.
11. RAA::conclusion="Reducto Ad Absurdum". This allows you to discard the last assumption (let) that you introduced.