Title: INCIDENCE GEOMETRIES
1INCIDENCE GEOMETRIES
2Contents
- Motivation
- Incidence Geometries
- Incidence Geometry Constructions
- Residuals, Truncations - Sections, Shadow Spaces
- Incidence Structures and Combinatorial
Configurations - Substructures, Symmetry and Duality
- Haar Graphs and Cyclic Configurations
- Algebraic Structures
- Euclidean Plane, Affine Plane, Projective Plane
- Point Configurations, Line Arrangements and
Polarity
- Pappus and Desarguers Theorem
- Existence and Countnig
- Coordinatization
- Combinatorial Configurations on Surfaces
- Generalized Polygons
- Cages and Combinatorial Configurations
- A Case Study The Gray Graph
- Another Case Study - Tennis Doubles
- Martinetti-Boben Theorem
31. Motivation
4Motivation
- When Slovenia joined the European Union it
obtained 7 seats in the Parliament of the
European Union. In 2004 the first elections to
the European Parliament in Slovenia were held. - There were 13 political parties (7 parliamentary
parties 1, 2, 3, 4, 5, 6, 7, and 6
non-parliamentary parties A, B, C, D, E, F)
competing for these seats. TV Slovenia decided to
cover the campain by hosting political parties in
6 TV shows a,b,c,d,e,f. - TV asked mathematicians to help them select the
guests in a fair way.
5Motivation
- With a little help from mathematicians TV came up
with the following schedule.
a b c d e f
A B C D E F
1 2 3 1 2 3
4 6 4 7 5 6
5 7
6Example TV coverage of EU parliamentary
elections in Slovenia
TV Shows Parties Parties Parties Parties
a A 1 4 5
b B 2 6
c C 3 4
d D 1 7
e E 2 5
f F 3 6 7
7Model
- We can model the above schedule as follows
- Let P 1,2,3,4,5,6,7,A,B,C,D,E,F
- Let L a,b,c,d,e,f
- Let I ½ P L such that
- (p,L) 2 I if and only if political party p
appears in the show L. - I (A,a), (1,a), (4,a), (5,a), ...
8Incidence structure
- An incidence structure C is a triple
- C (P,L,I) where
- P is the set of points,
- L is the set of blocks or lines
- I ? P ? L is an incidence relation.
- Elements from I are called flags.
9Levi Graph
- The bipartite incidence graph G(C) with black
vertices P, white vertices L and edges I is
known as the Levi graph of the structure C.
10Levi graph for the Election structure
A
a
- On the left there is the Levi graph for the
incidence structure of the media coverage of the
European Union Parliament elections in Slovenia. - Each parliamentary party appears twice and each
non-parliamentary party appears once. (check
valence!)
1
5
4
B
e
D
C
c
2
d
E
b
6
7
3
F
f
11Menger graph
- Given an incidence structure C (P,L,I) we say
that two points p and q are collinear, if there
is a line L that contains both of them. - Menger graph M(C).
- Vertices P
- p q if and only if p and q are collinear.
12Menger Graph from Levi Graph
- There is a simple procedure for computing M from
L. Take the pure graph power L(2). It is obtained
from L by taking the same vertex set and making
two vertices adjacent in L(2) if and only if they
are at distance two in L. Since L is bipartite
L(2). has (at least) two components. The one
defined on the black vertices (corresponding to
points of the incidence structure) is Menger
graph M. The other one is called dual Menger
graph.
13Menger graph for the Election structure
- On the left there is the Menger graph for the
incidence structure of the media coverage of the
European Union Parliament elections in Slovenia.
A
1
4
5
B
C
D
2
E
6
3
F
7
14Configuration Graph
- The configuration graph K is the complement of
the Menger graph. The dual configuration graph
is the complement of the dual Menger graph.
15Dual Configuration graph for the Election
structure
- On the left there is the dual configuration graph
for the incidence structure of the media coverage
of the European Union Parliament elections in
Slovenia.
f
e
c
d
a
b
16Dual Configuration graph for the Election
structure
- The Hamilton path abcdef in the dual
configuration graph guarantees that no political
party appears in two consecutive TV shows.
f
e
c
d
a
b
17Examples
- 1. Each graph G (V,E) is an incidence
structure P V, L E, (x,e) 2 I if and only if
x is an endvertex of e. - 2. Any family of sets F µ P(X) is an incidence
structure. P X, L F, I 2. - 3. A line arrangement L l1, l2, ..., ln
consisting of a finite number of n distinct lines
in the Euclidean plane E2 defines an incidence
structure. Let V denote the set of points from E2
that are contained in at least two lines from L.
Then P V, L L and I is the point-line
incidence in E2.
18Exercises 1
- N1. Draw the Levi graph of the incidence
structure defined by the complete bipartite graph
K3,3. - N2. Draw the Levi graph of the incidence
structure defined by the power set P(a,b,c). - N3. Determine the Levi graph of the incidence
structure, defined by an arrangement of three
lines forming a triangle in E2. - N4 Determine the Levi graph of the line
arrangement on the left.
192. Incidence Geometry
20Incidence geometry
- An incidence geometry (G,c) of rank k is a graph
G with a proper vertrex coloring c, where k
colors are used. - Sometimes we denote the geometry by (G,,T,c).
Here cV(G) ! T is the coloring and T k is
the number of colors, also known as the rank of
G. The relation is called the incidence. - T is the set of types. Note that only objects of
different types may be incident.
21Morphisms or representations
- Given two incidence geometries (G,,c,T) and
(G',,c',T') a pair (f,g) of mappings - f G ! G' and
- g T ! T' is called a morphism of geometries (or
representation) if the following is true - for any v 2 V(G) c'(f(v)) g(c(v)).
- for any u,v 2 V(G) if u v then either f(u)
f(v) or f(u) f(v).
22Special morphisms
- Some morphisms have nice properties and deserve
special attention. - We call a representation dimension-preserving if
- for any u,v 2 V(G) if u v then f(u) f(v).
- We call a representation faithful or strong if
- for any u,v 2 V(G) u v if and only if f(u)
f(v). - A faithful representation in which both f and g
are injective is called realization. - A morphism is an isomorphism if both f and g are
bijections and the inverse pair (f-1,g-1) is a
morphism too. - The image of a representation is geometry. The
image of a realization is isomorphic to the
original. - For stirng geometries we seek representations and
realizations in sets. Vertices are mapped to the
elements (or singletons) of S and the faces to
subsets of S. The incidence ui uj, i lt j is
represented by inclusions S(ui) µ S(uj).
23Automorphisms
- There are two types of automorphisms in a
geometry (G,,c,T). Aut0 G contains
type-preserving automorphisms. (g id). Aut G
contains all (extended) automorphisms. - In the case of string geometries we want the
linear order on T to be respected (or reversed).
In the case of extended automorphisms we speak of
dualities that map faces of rank r to rank n-r.
24Examples
- 1. Each incidence structure is a rank 2 geometry.
(Actualy, look at its Levi graph.) - 2. Each 3 dimensional polyhedron is a rank 3
geometry. There are three types of objects
vertices, edges and faces with obvious geometric
incidence. - 3. Each (abstract) simplicial complex is an
incidence geometry. Incidence is defined by
inclusion of simplices. - 4. Any complete multipartite graph is a geometry.
Take for instance K2,2,2, K2,2,2,2, K2,2, ..., 2.
The vertex coloring defining the geometry in each
case is obvious.
25Pasini Geometry
-
- Pasini defines incidence geometry (that we call
Pasini geometry) in a more restrictive way. - For k1, the graph must contain at least two
vertices V(G)gt1. - For kgt1
- G has to be connected,
- For each x ? V(G) the (k-1)-colored graph
(Gx,c), called residuum, induced on the neigbors
of x is a Pasini geometry of rank (k-1).
26String geometries
- A geometry G over the set of types T -1,0,1,
..., n is called a string geometry if the
following (1-2) is true (the elements of G are
called faces, faces of type 0 are called vertices
(or points), faces of type 1 are called edges (or
lines), faces of type n-1 are called facets.). It
is called pure string geometry if (1-3) is true. - There are exactly two improper faces u-1 2 V(G)
of type -1 and one element un 2 V(G) of type n
(both incident with every other face). The rest
are called proper faces. - If ui, uj, uk are elements of respective types
i lt j lt k and ui uj, uj uk, then ui uk. - Every collection of mutually incident faces U can
be extended to a sequence of (n2) mutually
incident faces. (In other words all chambers
have rank n2.)
27Incidence geometries of rank 2
- Incidence geometries of rank 2 are simply
bipartite graphs with a given black and white
vertex coloring. - Rank 2 Pasini geometries are in addition
connected and the valence of each vertex is at
least 2 d(G) gt1.
28Example of Rank 2 Geometry
- Graph H on the left is known as the Heawood
graph. - H is connected
- H is trivalent d(H) D(H) 3.
- H is bipartite.
- H is a Pasini geometry.
29Another View
- The geometry of the Heawood graph H has another
interpretation. - Rank 2. There are two types of objects in
Euclidean plane, say, points and curves. - There are 7 points, 7 curves, 3 points on a
curve, 3 curves through a point. - The corresponding Levi graph is H!
30In other words ...
- The Heawood graph (with a given black and white
coloring) is the same thing as the Fano plane
(73), the smallest finite projective plane. - Any incidence geometry can be interpeted in terms
of abstract points, lines. - If we want to distinguish the geometry
(interpretation) from the associated graph we
refer to the latter as the Levi graph of the
corresponding geometry. -
31Simplest Rank 2 Pasini Geometries
Cycle (Levi Graph)
- Simplest geometries of rank 2 in the sense of
Pasini are even cycles. For instance the Levi
graph C6 corresponds to the triangle. -
Triangle (Geometry)
32Rank 3
- Incidence geometries of rank 3 are exactly
3-colored graphs. - Pasini geometries of rank 3 are much more
restricted. Currently we are interested in those
geometries whose residua are even cycles. - Such geometries correspond to Eulerian surface
triangulations with a given vertex 3-coloring.
33Flag System as Geometries
- Any flag system ? µ V E F defines a rank 3
geometry on X V t E t F. There are three types
of elements and two distinct elements of X are
incicent if and only if they belong to the same
flag of ?.
34Self-avoiding maps
- Recall that a map is self-avoiding if and only if
neither the skeleton of the map nor the skeleton
of its dual has a loop.
35Self-avoiding maps as Geometries of rank 4
- Consider a generalized flag system ? µ V E F
P that defines a rank 4 geometry on X V t E t
F t P. - There are four types of elements and two distinct
elements of X are incident if and only if they
belong to the same flag of ?. - We may take any self-avoiding map M and the four
involutions ?0,?1,?2 and ?3 and define a geometry
as above.
36Exercises 2
- N1. Prove that the Petrie dual of a self-avoiding
map is self-avoiding. - N2. Prove that any operation Du,Tr,Me,Su1, ... of
a self-avoiding map is self-avoiding. - N3. Prove that BS of any map is self-avoiding.
- N4. Show that any self-avoiding map may be
considered as a geometry of rank 4 (add the
fourth involution).
37Homework 2
- H1 Describe the rank 4 geometry of the projective
planar map on the left.
383. Incidence Geometry Constructions
39Geometries from Groups
- Let G be a group and let G1,G2,...,Gk be a
family of subgroups of G. - Form the cosets xGt, t 2 1,2, ..., k.
- An incidence geometry of rank k is obtained as
follows - Elements of type t 2 1,2,...,k are the cosets
xGt. - Two cosets are incident xGt yGs if and only if
xGt Å yGs ¹ .
40Q The Quaternion Units
Q 1 -1 i -i j -j k -k
1 1 -1 i -i j -j k -k
-1 -1 1 -i i -j j -k k
i i -i -1 1 k -k -j j
-i -i i 1 -1 -k k j -j
j j -j k -k -1 1 i -i
-j -j j -k k 1 -1 -i i
k k -k j -j -i i -1 1
-k -k k -j j i -i 1 -1
41Geometry from Quaternions
- Example Q 1,-1,i,-i,j,-j,k,-k.
- Gi 1,-1,i,-i, Gj 1,-1,j,-j, Gk
1,-1,k,-k.
42Quaternions - Continiuation
j,k
- The Levi graph is an octahedron.
- Labels on the left
- i 1,-1,i,-i
- j,k j,-j,k,-k, etc.
j
k
i
i,j
i,k
43Quaternions Examle of Rank 4 Geometry.
j,k
- Levi graph was an octahedron.
- Notation
- i 1,-1,i,-i
- j,k j,-j,k,-k, etc.
- If we add the sugroup G0 1,-1, four more
cosets are obtained - Additional notation
- 1 1,-1,ii,-i, etc.
k
j
k
1
j
i
i
i,k
i,j
44Reyes Configuration
- Reyes configuration of points, lines and planes
in 3-dimensional projective space consists of - 8 1 3 12 points (3 at infinity)
- 12 4 16 lines
- 6 6 12 planes.
P12 L16 S12
P12 - 4 6
L16 3 - 3
S12 6 4 -
45Theodor Reye
- Theodor Reye (1838 - 1919), German Geometer.
- Known for his book
Geometrie der Lage (1866 and 1868). - Published his famous configuration in 1878.
- Posed the problem of configurations.
46Centers of Similitude
- We are interested in tangents common to two
circles in the plane. - The two intersections are called the centers of
similitudes of the two circles. The blue center
is called the internal, the red one is the
external center. - If the radii are the same, the external center is
at infinity.
47Reyes Configuration -Revisited
- Reyes configuration can be obtained from centers
of similitudes of four spheres in three space
(see Hilbert ...) - Each plane contains a complete quadrangle.
- There are 2 C(4,2) 2 4
3/2 12 points.
48Exercises 3-1
- N1. Consider the geometry defined by Z3 and Z5 in
Z15. Draw its Levi graph. - N2. Draw the Levi graph of the geometry defined
by all non-trivial subgroups of the symmetric
group S3. - N3. Draw the Levi graph of the geometry defined
by all non-trivial subgroups of the group Z23.
49Exercises 3-2
- N4. Let there be three circles in a plane giving
rise to 3 internal and 3 external centers of
similitude. Prove that the three external centers
of similitude are colinear.
504. Residuals, Truncations - Sections, Shadow
Spaces
51Residual geometry
- Each incidence geometry
- G (G, , T,c)
- (G,) a simple graph
- c, proper vertex coloring,
- T collection of colors.
- c V(G) ! T
- Each element x 2 V(G) determines a residual
geometry Gx. defined by an induced graph defined
on the neighborhood of x in G.
G
Gx
x
52Flags and Residuals
- In an incidence geometry G a clique on m vertices
(complete subgraph) is called a flag of rank m. - Residuum can be definied for each flag F ½ V(G).
G(F) ÅG(x) Gx x 2 F.
53Chambers and Walls
- A maximal flag (flag of rank T is called a
chamber. A flag of rank T-1 is called a wall. - To each geometry G we can associate the chamber
graph - Vertices chambers
- Two chambers are adjacent if and only if they
share a common wall. - (See Egon Shulte, ..., Tits systems)
54The 4-Dimensional Cube Q4.
0010
0001
0000
0100
1000
55Hypercube
- The graph with one vertex for each n-digit binary
sequence and an edge joining vertices that
correspond to sequences that differ in just one
position is called an n-dimensional cube or
hypercube. - v 2n
- e n 2n-1
564-dimensional Cube.
0110
0010
0111
1110
0011
1010
1111
1011
0001
1101
1001
0000
0100
1100
1000
574-dimensional Cube and a Famous Painting by
Salvador Dali
- Salvador Dali (1904 1998) produced, in 1954,
the Crucifixion (Metropolitan Museum of Art, New
York) in which the cross is a 3-dimensional net
of a 4-dimensional hypercube.
584-dimensional Cube and a Famous Painting by
Salvador Dali
- Salvador Dali (1904 1998) produced in 1954, the
Crucifixion (Metropolitan Museum of Art, New
York) in which the cross is a 3-dimensional net
of a 4-dimensional hypercube.
59The Geometry of Q4.
- Vertices (Q0) of Q4 16
- Edges (Q1)of Q4 32
- Squares (Q2) of Q4 24
- Cubes (Q3) of Q4 8
- Total 80
- The Levi graph of Q4 has 80 vertices and is
colored with 4 colors.
60Residual geometries of Q4.
V E S Q3.
G(V) - 4 6 4
G(E) 2 - 3 3
G(S) 4 4 - 2
G(Q3) 8 12 6 -
61Truncations or Sections
- Given a geometry G (V,,T,c) and a subset of
types J µ T, define a J-section G/J of G as the
geometry H (U,,J,c), where U v 2 V c(v) 2
J and H is the induced subgraph of G.
62Quaternions Example of Rank 4 Geometry - Section
j,k
j
k
i
i,j
i,k
Rank 3 section
Rank 4 geometry
63Shadow Spaces
- Given a geometry G (V,,T,c) and J µ T we may
define an incidence structure Spa(G,J) whose
points are J-flags and the blocks are composed of
those sets of J-flags that belong to the residual
geometry G(F) for some flag F from the original
geometry G.
64Shadow Spaces - An Example
3
4
- Let us denote the types
- I g,r,b.
- Let J r,b. There are three J-flags 26, 45
and 56. The set system for the shadow space - 45,26,45,56,26,56.
- For J g,b we get three flags
- 16,14,34
- The set system for the shadow space
- 16,34,14,16,14,34
5
6
1
2
65Shadow spaces of Maps
- For maps as rank 3 geometries the notion of
shadow spaces gives rise to an interesting
interpretation. There are three types of objects
v,e,f. - Hence, there are 7 types of shadow spaces
- v - primal id
- e - medial Me
- f - dual Du
- v,e - truncation Tr
- v,f - Me Me
- e,f - leapfrog Le
- v,e,f- Co
66Shadows - Example
- Our map is a prism. All flags (structured by
type) - ,
- 1,2,3,4,5,6
- a,b,c,d,e,f,g,h,i
- A,B,C,D,E
- 1a,1d,1e,2a,2b,2f,3e,3f,3i,4c,4d,4h,5b,5c,5g,6g,6h
,6i - 1A,1B,1C,2A,2B,2D,3B,3C,3D,4A,4C,4E,5A,5D,5E,6C,6D
,6E - aA,aB,bA,bD,cA,cE,dA,dC,eB,eC,fB,fD,gD,gE,hC,hE,iC
,iD - 1aA,1aB,1dA,1dC,1eB,1eC,2aA,2aB,2bA,2bD,2fB,2fD,3e
B,3eC,3fB,3fD,3iC,3iD,4cA,3cE,4dA,4dC,4hC,4hE,5bA,
5bD,5cA,5cE,5gD,5gE,6gD,6gE,6hC,6hE,6iC,8iD
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
67Shadows - Example - Primal
- Our map is a prism. T v,e,f
- J v
- J-flags 1, 2, 3, 4, 5, 6
- Sets 12, 13, 14, 23, 25, 36, 45, 46, 56, 123,
456, 1245, 1346, 2356.
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
68Shadows - Example - Dual
- Our map is a prism. T v,e,f
- J f
- Flags A,B,C,D,E
- Sets AB, AC, AD, AE, BC, BD, CD, CE, DE, ABC,
ABD, BCD, CDE, ACE, ADE.
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
69Shadows - Example - Medial
- Our map is a prism. T v,e,f
- J e
- Flags a,b,c,d,e,f,g,h,i
- Sets ae,ab,ad,af,bc,bf,bg,cd,cg,ch,de,dh,ef,ei,f
i,gh,gi,hi, aef, bfgi, dehi, abcd,cgh.
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
70Shadows - Example - Truncation
- Our map is a prism. T v,e,f
- J v,e
- Flags 1a,1d,1e,2a,2b,2f,3e,3f,3i,4c,4d,4h,5b,5c,5
g,6g,6h,6i - Sets 1a,1d,1e,2a,2b,2f,3e,3f,3i,4c,4d,4h,5b,5c,5g
,6g,6h,6i - ...
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
71Posets
- Let (P,) be a poset. We assume that we add two
special (called trivial) elements, 0, and 1, such
that for each x 2 P, we have 0 x 1.
72Ranked Posets
- Note that a ranked poset (P,) of rank n has the
property that there exists a rank function rP !
-1,0,1,...,n, r(0) -1, r(1) n and if y
covers x then r(y) r(x) 1. (All linear chains
have the same length) - If we are given a poset (P, ) with rank function
r, then such a poset defines a natural incidence
geometry. - V(G) P.
- x y if and only if x lt y.
- c(x) r(x). Vertex color is just the rank.
- The corresponding geometry is a string geometry.
There is a one-to-one correspondence between the
two concepts.
73Intervals in Posets
- Let (P,) be a poset.
- Then I(x,z) y x y z is called the
interval between x and z. - Note that I(x,z) is empty if and only if x z.
- I(x,z) is also a ranked poset with 0 and 1.
74Connected Posets.
- A ranked poset (P,) wih 0 and 1 is called
connected, if either rank(P) 1 or for any two
non-trivial elements x and y there exists a
sequence x z0, z1, ..., zm y, such that there
is a path avoiding 0 and 1 in the Levi graph from
x to y and the rank function is changed by 1 at
each step of the path.
75Abstract Polytopes
- Peter McMullen and Egon Schulte define abstract
polytopes as special ranked posets. - Their definition is equivalent to the following
- (P,) is a ranked poset with 0 and 1 (minimal and
maximal element) - For any two elements x and z, such that r(z)
r(x)2, x lt z there exist exactly two elements
y1, y2 such that x lt y1 lt z, x lt y2 lt z. - Each section is connected.
- Note that abstract poytopes are a special case of
posets but they form also a generalization of the
convex polytopes.
76Convex vs abstract polytopes
- To each convex polytope we may associate an
abstract polytope. For instance, the tetrahedron
- 0
- 4 vertices v1, v2, v3, v4.
- 6 edges e1, e2, ..., e6,
- 4 faces t1,t2,t3, t4
- 1
- e1 v1v2, e2 v1v3, e3 v1v4, e4 v2v3, e5
v2v4, e6 v3v4. - t4 v1v2v3, t1 v2v3v4, t3 v1v2v4, t2
v1v3v4.
77The Poset
1
- In the Hasse diagram we have the following local
picture
t2
t1
t3
t4
e2
e3
e4
e5
e1
e6
v2
v1
v3
v4
0
78Diagram geometries
- For any incidence geometry G(V,,T,c) we usually
study for each pair i,j 2 T the section
(truncation) of rank two G/(i,j). We
deliberatly make distinction between G/(i,j) and
its dual G/(j,i). Sometimes each connected
component of G/(i,j) has the same structure. This
is indicated by a diagram. A diagram in an
edge-labeled graph on the vertex set T, where the
lables indicate the structure of each section.
79String diagram geometries
- The edge between i an j is omitted if and only if
G/(i,j) is a generalized digon. This means that
each connected component is a complete bipartite
graph. - G is called a string diagram geometry if the
corresponding diagram has a shape of a path (or
union of paths). - Example Each abstract polytope is a string
diagram geometry.
80The Grassmann graph
- Let G(V,,T,c) be an incidence geometry and let i
2 T be a type. Then we define the Grassmann graph
G(i) to on the vertex set V(i) v 2 V c(v)
i and two vertices u and v are adjacent in G(i)
if an only if for each j ? i there exists an w 2
V(j) such that u w and w v (in the original
geometry.) - Example For instance, in the case of rank two
geometries, the Grassmann graphs are exactly the
Menger graph and the dual Menger graph.
81Exercises 4-1
- N1. Our map is a prism. I v,e,f
- For each set of type
- J v,f
- J e,f
- J v,e,f
- determine the shadow space.
5
4
c
E
g
h
6
d
b
C
D
i
3
f
e
B
2
a
1
A
82Exercises 4-2
- N2. Repeat the analysis of previous two slides
for the simplex K5. - N3. Repeat the analysis of the previous two
slides for the generalized octahedron K2,2,2,2.
83Exercises 4-3
- N4 Determine all residual geometries of Reyes
configuration - N5 Determine all residual geometries of Q4.
- N6 Determine all residual geometries of the
Platonic solids. - N7 Determine the Levi graph of the geometry for
the group Z2 Z2 Z2, with three cyclic
subgroups, generated by 100, 010, 001,
respectively.
84Exercises 4-4
- N18 Determine the posets and Levi graphs of
each of the polytopes on the left. - Solution for the haxagonal pyramid
- 0
- 7 vertices v0, v1, v2, ..., v6.
- 12 edges e1, e2, ..., e6, f1, f2, ..., f6
- 7 faces h,t1,t2,t3,.., t6
- 1
- e1 v1v2, e2 v2v3, e3 v3v4, e4 v4v5, e5
v5v6, e6 v6v1, f1 v1v0, f2 v2v0,f3 v3v0,
f4 v4v0, f5v5v0, f6 v6v0. - h v1v2v3v4v5v6,
- t1 v1v2v0, t2 v2v3v0, t3 v3v4v0, t4
v4v5v0, t5 v5v6v0, t6 v6v1v0,
855. Incidence Structures
86Incidence structure
- An incidence structure C is a triple
- C (P,L,I) where
- P is the set of points,
- L is the set of blocks or lines
- I ? P ? L is an incidence relation.
- Elements from I are called flags.
- The bipartite incidence graph G(C) with black
vertices P, white vertices L and edges I is
known as the Levi graph of the structure C.
87(Combinatorial) Configuration
- A (vr,bk) configuration is an incidence structure
C (P,L,I) of points and lines, such that - v P
- b L
- Each point lies on r lines.
- Each line contains k points.
- Two lines intersect in at most one point.
- Warning Levi graph is semiregular of girth ? 6
88Symmetric configurations
- A (vr,bk) configuration is symmetric, if
- v b (this is equivalent to r k).
- A (vk,vk) configuration is usually denoted by
(vk).
89Small Configurations
- Triangle, the only (32) configuration.
- Pasch configuration (62,43) and its dual Perfect
Quadrangle (43,62) have the same Levi graph.
906. Substructures, Symmetry and Duality
91Substructures
- An incidence structure C (P, L,I) is a
substructure of an incidence structure C (P,
L,I), C µ C, if P µ P, L µ L and I µ I.
92Duality
- Each incidence structure C (P,L,I) gives rise
to a dual structure Cd (L,P,Id) with the role
of points and lines reversed and keeping the
incidence. - The structures C and Cd share the same Levi graph
with the roles of black and white vertices
reversed.
93Self-Duality and Automorphisms
- If C is isomorphic to its dual Cd , it is said
that C is selfdual, the corresponding
isomorphism is called a (combinatorial) duality. - A duality of order 2 is called (combinatorial)
polarity. - An isomorphism mapping C to itself is called an
automorphism or (combinatorial) collinearity.
94Automorphisms and Antiautomorphisms
- Automorphisms of the incidence structure C form a
grup that is called the group of automorphisms
and is denoted by Aut0C. - If automorphisms and dualities (antiautomorphisms)
are considered together as permutations, acting
on the disjoint union P ? L, we obtain the
extended group of automorphism Aut C. - Warning If C is disconnected there may be
mixed automorphisms.
95Graphs and Configurations
- The Levi graph of a configuration is bipartite
and carries complete information about the
configuration. - Assume that C is connected. The extended group of
automorphisms AutC coincides with the group of
automorphisms of the Levi graph L ignoring the
vertex coloring, while Aut0C stabilises both
colors.
96Examples
- 1. Each graph G (V,E) is an incidence
structure P V, L E, (x,e) 2 I if and only if
x is an endvertex of e. - 2. Any family of sets F µ P(X) is an incidence
structure. P X, L F, I 2. - 3. A line arrangement L l1, l2, ..., ln
consisting of a finite number of n distinct lines
in the Euclidean plane E2 defines an incidence
structure. Let V denote the set of points from E2
that are contained in at least two lines from L.
Then P V, L L and I is the point-line
incidence in E2.
97Exercises 6
- N1 Draw the Levi graph of the incidence
structure defined by the complete bipartite graph
K3,3. - N2 Draw the Levi graph of the incidence
structure defined by the powerset P(a,b,c). - N3 Determine the Levi graph of the incidence
structure, defined by an arrangemnet of three
lines forming a triangle in E2.
987. Haar Graphs and Cyclic Configurations
99Haar graph of a natural number
-
- Let us write n in binary
- n bk-12k-1 bk-2 2k-2 ... b12 b0
- where B(n) (bk-1, bk-2, ..., b1, b0), bk-1
1are binary digits of n. Graph H(n) H(k n),
called the Haar graph of the natural number n,
has vertex set ui, vi, i0,1,...,k-1. Vertex ui
is adjacent to vij, if and only if bj 1
(arithmetic is mod k).
100Remark
-
- When defininig H(n) we assumed that k is the
number of binary digits of n. In general, for
H(kn) one can take k to be greater than the
number of binary digits. In such a case a
different graph is obtained!
101Example
- Determine H(37).
- Binary digits
- B(37) 1,0,0,1,0,1
- k 6.
- H(37) H(637) is depicted on the left!
102Dipoles qn
- The dipole qn has two vertices, joined by n
parallel edges. If we want to distinguish the two
vertices, we call one black, the other one white.
On the left we see q5. - Each dipole is a bipartite graph. Therefore each
of its covering graphs is a bipartite graph. - In particular q3 is a cubic graph also known as
the theta graph q.
103Cyclic covers over a dipole
- Each Haar graph is a cyclic cover over a dipole.
One can use the following recipe - H(37) is determined by a natural number 37, or,
equivalently by a binary sequence(1 0 0 1 0 1). - The length is k6, therefore the group Z6.
- The indices are written below
- (1 0 0 1 0 1)
- (0 1 2 3 4 5)
- The 1s appear in positions 0, 3 in 5. These
numbers are used as voltages for H(37).
0
3
5
Z6
104Connected Haar graphs
- Graph G is connected if there is a path between
any two of its vertices. - There exist disconnected Haar graphs, for
instance H(10). - Define n to be connected, if the corresponding
Haar graph H(n) is connected. - Disconnected numbers 2,4,8,10,16,32,34,36,40,42,6
4...
105The Mark Watkins Graph
- The cubic Haar graph H(536870930) has an
interesting property. 536870930 is the smallest
connected number that is cyclically equivalent to
no odd number. - Recall that two sets S,T µ Zn are cyclically
equivalent if there exists a 2 Zn and b 2 Zn
such that S aT b (mod n).
106Girth of Connected Haar graphs
- K2 is the only connected 1-valent Haar graph.
- Even cycles C2n are connected 2-valent Haar
graphs. - Theorem Let H be a connected Haar graph of
valence d gt 2. Then either girth(H) 4 or
girth(H) 6.
107Cyclic Configurations
- A symmetric (vr) configuration determined by its
first column s of the configuration table where
each additional column is obtained from s by
addition (mod m) is called a cyclic
configuration Cyc(ms). - The left figure depicts a cyclic Fano
configuration Cyc(71,2,4) Cyc(70,1,3).
a b c d e f g
k 1 2 3 4 5 6 0
k1 2 3 4 5 6 0 1
k3 4 5 6 0 1 2 3
108Connection to Haar graphs
- Theorem A symmetric configuration (vr), r 1
is cyclic, if and only if its Levi graph is a
Haar graph with girth ¹ 4. - Corollary Each cyclic configuration is point-
and line-transitive and combinatorially
self-dual. - Corollary Each cyclic configuration (vr), r gt 2
contains a triangle. - Question Does there exist a cyclic configuration
that is not combinatorially self-polar?
109Problem
- Study cyclic configurations with respect to flag
orbits. - Example On the left we see the smallest
0-symmetric graph Haar(261) on 18 vertices. It is
the Levi graph of the cyclic (93) configuration
having 3 flag orbits.
110Exercises 7-1
- The graph on the left is the so-called Heawood
graph H. Prove - N1 H is bipartite
- N2 H is a Haar graph. (Find n!)
- N3 Determine H as a cyclic cover over q3..
- N4 Prove that H has no cycle of length lt 6.
- N5 Prove that H is the smallest cubic graph of
girth 6. - N6 Find a hexagonal torus embedding of H .
- N7 Determine the dual of the embedded H.
111Exercises 7-2
- N8 Prove that each 2m is a disconnected number.
- N9 Show that the Möbius-Kantor graph G(8,3) is a
Haar graph of some number. Which number is that? - N10 () Determine all generalized Petersen
graphs that are Haar graphs of some natural
number. - N11 Show that some Haar graphs are circulants.
- N12 Show that some Haar graphs are
non-circulants.
112Exercises 7-3
- N13 Prove that each Haar graph is vertex
transitive. - N14 Prove that each Haar graph is a Cayley graph
for a dihedral group. - N15 Prove that there exist bipartite Cayley
graphs of dihedral groups that are not Haar
graphs (such as the graph on the left).
113Exercises 7-4
- N16 The numbers n and m are cyclically
equivalent, if the binary string of the first
number can be cyclically transformed to the
binary string of the second number. This means
that the string can be cyclically permuted,
mirrored or multiplied by a number relatively
prime with the string length. - N17 The numbers n and m are Haar equivalent, if
their Haar graphs are isomorphic H(n) H(m). - N18 Prove that cyclic equivalence implies Haar
equivalence. - N19 Determine all numbers that are cyclically
equivalent to 69. - N20 Use a computer to show that 137331 and
143559 are Haar equivalent, but are not
cyclically equivalent.
114Exercises 7-5
- N21 Show that each Haar graph of an odd number
H(2n1) is hamiltonian and therefore connected.
115Homework 7
- Use Vega to explore the edge-orbits of cyclic
Haar graphs. - H1. Find an example of a cubic Haar graph that
has 1,2, or 3 edge orbits. - H2. Find an example of a quartic Haar graph that
has 1, 2, 3, or 4 edge orbits. Study the graphs
with 2 edge orbits.
1168. Algebraic Structures
117Real Numbers R.
- Let us review the structure of the set of real
numbers (real line) R. - In particular, consider addition and
multiplication . - (R,) forms an abelian group.
- (R,) does not form a group. Why?
- (R,,) forms a (commutative) field.
118Real Numbers R. - Exercises
- N43 Write down the axioms for a group, abelian
group, a ring and a field. - N44 What algebraic structure is associated with
the integers (Z,,)? - N45 Draw a line and represent the numbers R.
Mark 0, 1, 2, -1, ½, p.
119A Skew Field K
- A skew field is a set K endowed with two
constants 0 and 1, two unary operations - - K ! K,
- K ! K,
- and with two binary operations
- K K ! K,
- K K ! K,
- satisfying the following axioms
- (x y) z x (y z) associativity
- x 0 0 x x neutral element
- x (-x) 0 inverse
- x y y x commutativity
- (x y) z x (y z). associativity
- (x 1) (1 x) x unit
- (x x) (x x) 1, for x ¹ 0. inverse
- (x y) z x z y z. left
distributivity - x (y z) x y y z. right
distributivity - A (commutative) field satisfies also
- x y y x.
120Examples of fields and skew fields
- Reals R
- Rational numbers Q
- Complex numbers C
- Quaterions H (non-commutative!! Will consider
briefly later!) - Residues mod prime p Fp
- Residues mod prime power q pk Fq (more
complicated, need irreducible poynomials!!Will
consider briefly later!)
121Complex numbers C
- a a bi 2 C
- a a bi
- b c di 2 C
- ab (ac bd) (bc ad)i
- b ¹ 0, a/b (ac bd) (bc ad)i/c2 d2
- a-1 (a bi)/(a2 b2)
122Quaternions H.
- Quaternions form a non-commutative field.
- General form
- q x y i z j w k., x,y,z,w 2 R.
- i 2 j 2 k 2 -1.
- q x y i z j w k.
- q x y i z j w k.
- q q (x x) (y y) i (z z) j (w
w) k. - How to define q .q ?
- i.j k, j.k i, k.i j, j.i -k, k.j -i,
i.k -j. - q.q (x y i z j w k)(x y i z j
w k)
123Quaternions H. - Exercises
- N46 There is only one way to complete the
definition of multiplication and respect
distributivity! - N47 Represent quaternions by complex matrices
(matrix addition and matrix multiplication)!
Hint q a b -b a. (We are using Matlab
notation).
a b
-b a
124Residues mod n Zn.
- Two views
- Zn 0,1,..,n-1
- Define on Z
- x y x y cn
- Zn Z/
- (Zn,) is an abelian group, namely a cyclic
group. Here is taken mod n!!!
125Example (Z6, ).
0 1 2 3 4 5
0 0 1 2 3 4 5
1 1 2 3 4 5 0
2 2 3 4 5 0 1
3 3 4 5 0 1 2
4 4 5 0 1 2 3
5 5 0 1 2 3 4
126Example (Z6, ).
0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 1 2 3 4 5
2 0 2 4 0 2 4
3 0 3 0 3 0 3
4 0 4 2 0 4 2
5 0 5 4 3 2 4
127Example (Z6\0, ).
It is not a group!!! For p prime, (Zp\0, )
forms a group (Zp, ,) Fp.
1 2 3 4 5
1 1 2 3 4 5
2 2 4 0 2 4
3 3 0 3 0 3
4 4 2 0 4 2
5 5 4 3 2 4
128Vector space V over a field K
- V V ! V (vector addition)
- . K V ! V (scalar multiple)
- (V,) abelian group
- (l m)x l x m x
- 1.x x
- (l m).x l(m x)
- l.(x y) l.x l.y
1299. Euclidean Plane, Affine Plane, Projective Plane
130Euclidean plane E2 and real plane R2
- R2 (x,y) x,y 2 R
- R2 is a vector space over R. The elements of R2
are ordered pairs of reals. - (x,y) (x,y) (xx,yy)
- l(x,y) (l x,l y)
- We may visualize R2 as an Euclidean plane (with
the origin O).
131Subspaces
- One-dimensional (vector) subspaces are lines
through the origin. (y ax) - One-dimensional affine subspaces are lines. (y
ax b)
y ax b
y ax
o
132 Three important results
- Thm1 Through any pair of distinct points passes
exactly one affine line. - Thm2 Through any point P there is exactly one
affine line l that is parallel to a given affine
line l. - Thm3 There are at least three points not on the
same affine line. - Note parallel not intersecting or identical!
133Affine Plane
- Axioms
- A1 Through any pair of distinct points passes
exactly one line. - A2 Through any point P there is exactly one line
l that is parallel to a given line l. - A3 There are at least three points not on the
same line. - Note parallel not intersecting or identical!
134Examples
- Each affine plane is an incidence structure C
(P,L,I) of points and lines. - Let K be a field, then K2 has a structure of an
affine plane. - K Fp.
- Determine the number of points and lines in the
affine plane A2(p) Fp2.
135Parallel Lines
- Parallel lines l m define an equivalence
relation on the set of lines. - l l
- l m ) m l
- l m, m n ) l n.
136A pencil of parallel lines
- An equivalence class of parallel lines is called
a pencil of parallel lines. - Thm. Each pencil of parallel lines defines an
equivalence relation on the set of lines.
137Ideal points and Ideal line
- Each pencil of parallel lines defines a new
point, called an ideal point (or a point at
inifinity.) New point is incident with each line
of the pencil. - In addition we add a new ideal line (or line at
infinity)
138Extended Plane
- Let A be an arbitrary affine plane. The incidence
structure obtained from A by adding ideal points
and ideal lines is called the extended plane and
is denoted by P(A).
- Theorem. Let C be an extended plane obtained from
any affine plane. The following holds - T1. For any two distinct points P and Q there
exists a unique line l connecting them. - T2. For any two distinct lines l and m there
exists a unique point P in their intersection. - T3. There exist at least four points P,Q,R,S such
that no three of them are colinear.
139Projective Plane
- Axioms for the Projective Plane. Let C be an
incidence structure of points and lines that
satisfies the following axioms - P1. For any two distinct points P and Q there
exists a unique line l connecting them. - P2. For any two distinct lines l and m there
exists a unique point P in their intersesction. - P3. There exist at least four points P,Q,R,S such
that no three of them are colinear.
140Linear Transformations
- In a vector space the important mappings are
linear transformations - L(l x m y) l L(x) m L(y). L-1 exists.
- L can be represented by a nonsingular square
matrix.
141Semi Linear Transformations
- A semi linear transformation is more general
- L(lx m y) f(l) L(x) f(m) L(y). L-1 exists,
f K ! K is an automorphism of K.
142Affine Transformations
- In an affine plane the important mappings are
affine transformations (affinities). - An affine transformation maps sets of collinear
points to collinear points. - Each affine transformation is of the form A(x)
c, where A is a semilinear transformation.
143Projective plane from R3
- Consider the incidence structure defined by
1-dimensional and 2-dimensional subspaces of R3
where the incidence is defined by inclusion. - Call 1-dimensional subspaces points and
2-dimensional subspaces lines.
144Homogeneous Coordinates
- Let (a,b,c) ¹ (0,0,0) be a point in R3. There is
exactly one line through the origin passing
through (a,b,c). Hence a projective point can be
represented by (a,b,c). However, for any l ¹ 0
the same projective point can be represented by
(l a, l b, l c). - That is why (a,b,c) are called homogeneous
coordinates.
145Unit sphere model
- Take a unit sphere in R3.
- Let pairs of antipodal points be projective
points. - Let big circles be projective lines.
- Prove that this system is a model for a
projective plane.
146Exercises 9-1
- N1. Conditions 1. and 2. are true for any
incidence structure. (Prove it!) - N2 Prove condition 3 for affine planes and find
a counter-example for general incidence
structure. - N3. Prove that this structure satisfies all three
axioms for the projective plane. - N4 Prove that in R, Q, Fp, (p- prime) there are
no nontrivial automorphisms.
147Exercises 9-2
- N5 Prove that z a z (conjugate) is an
automorphism of C. - N6 Go to the library or the internet and find a
reference to the group of authomorphisms of the
complex numbers C and the quaternions H. - N7 Determine the size of the group of
automorphisms of Fq, for q pk, a power of a
prime.
14810. Point Configurations, Line Arrangements,
Polarity
149Point Configuration
- A point configuration in R2 is a collection of
points affinely spanning R2. - In other words not all points are collinear.
150Line Arrangement
- A line arrangement is a partitioning of the plane
R2 into connected regions (cells, edges, and
vertices) induced by a finite set of lines.
151Area of a Triangle
- Area of the green trapezoid
- A12 (1/2)(y2 y1) (x2 x1)
- In the same way
- A23 (1/2)(y2 y3) (x3 x2)
- A13 (1/2)(y3 y1) (x3 x1)
- Area of the triangle
- T A12 A23 A13.
P2(x2,y2)
y2
y1
P1(x1,y1)
y3
P3(x3,y3)
O
x2
x1
x3
152Area of a Triangle
P2(x2,y2)
y2
y1
P1(x1,y1)
y3
P3(x3,y3)
O
x2
x1
x3
153Triple of Collinear Points
P2(x2,y2)
y2
y1
P1(x1,y1)
y3
P3(x3,y3)
O
x2
x1
x3
The points P1(x1,y1), P2(x2,y2), P3(x3,y3), are
collinear if and only if T 0.
154Point Configurations Line Arrangements
- Each point configuration S gives rise to a line
arrangement A(S). The lines are determined by all
pairs of points. - Another line arrangement A3(S) is determined by
triples of collinear points.
155Polarity with Respect to a Circle
p
- Let us consider the extended plane and a circle K
in it. There is a mapping from points to lines
(and vice versa). p p a P. - p polar
- P pole
P
p
P
p
P
156Polarity with respect to the unit circle
- Given P(a,b) the equation of the polar is
- p y (-a/b)x (1/b)
- p by ax 1
- In general Circle K(p,qr)
- p y(b-q) x(a-p) p(a-p) q(b-q) r2.
- Given
- p y kx n
- P(a,b)
- a -k/n
- b 1/n
- In general
- a p-kr2/(kp n q)
- b q r2/(kp n q)
157Natural Parameters p,q,r
- For a given point configuration S the center of
the circle(p,q) is determined as the barycenter
of S while the radius is given as the average
distance from the center.
158Polarity in General
- A general polarity is defined with respect to a
conic section (ellipse, hyperbola, or parabola).
159Polar Duality of Vectors and Central Planes in R3.
- A polar duality is a mapping associating a vector
v 2 R3 with an oriented central plane having v as
its normal vector and vice versa.
160A Standard Affine Polar-Duality
- A standard affine polar duality is a mapping
between non-vertical lines and points of R2
associating the non-vertical line y ax b with
the point (a,-b) and vice versa.
161Polar Duality of Points and Lines in the Affine
Space.
- General rule Take a polar-duality of vectors and
central planes and consider the intersetion with
some affine plane in R3 .
162Homogeneous Coordinates
- Take the affine plane z 1. A point with
Euclidean coordinates (x,y) can be assigned the
homogeneous coordinates (x,y,1). Ideal points get
homogeneous coordinates (x,y,0).
(z0x0,z0y0,z0)
(x0,y0,1)
(x0,y0)
163Equation of a plane through the origin
- Recall general plane
- ax by cz d.
- Equation of a plane through the origin
- ax by cz 0-
- Another meaning
- (x,y,z) homogeneous coordinates of a projective
point - a,b,c homogeneous coordinates of a projective
line.
164Point on a Line
- Let (a,b,c) be homogeneous coordinates of a point
P and let A,B,C be homogeneous coordinates of a
line p. - Then P lies on p if and only if aA bB cC 0.
- Let P(a,b,c) and P(a,b,c). The equation of a
line through P Æ P. is defined by the cross
product A,B,C (a,b,c) (a,b,c). - Similarly we get the intersection of two lines.
165Example
- Polarity of a point configuration consisting of
the points of a 10 10 grid. - Parameters of the circle are determined
automatically.
166Star Polygons (n/k).
- By (n/k) we denote star polygons.
- Note that each of them defines an incidence
structure. in which the points are the vertices
and intersections while the lines are the edges
of a polygon.
3/1
4/1
5/1
5/2
6/1
6/2
7/1
7/2
7/3
167Fano Plane
- We obtain the Fano plane from F23. There are
obviously 7 (non-zero) points Any pair of points
defines a unique line that contians exactly one
additional point.
0 0 1 0 1 1 1
0 1 0 1 0 1 1
1 0 0 1 1 0 1
168Exercises 10-1
- A polarity maps a point configuration to a line
arrangement and vice versa. - N1Take an equilateral triangle ABC with sides
a,b,c. Find a polarity, such that a a A, b a B
and c a C. - N2 Determine the polar figure of point
configuration determined by the vertices of a
regular n-gon with respect to its inscribed
circle. - N3 Determine the polar of an ideal point and the
pole of the ideal line. -
169Exercises 10-2
- N4 Determine the number of points and lines of
the incidence structure defined by the star
polygon 5/2. - N5() Determine the number of points and lines
of the incidence structure determined by the star
polygon n/k.
170Homework 10
- H1 Prove that the number of points on each line
of any finite projective plane P is constant, say
q1. Then q is called the order of P. - H2 Which axioms of a projective planes are valid
for a near-pencil N(n)?
17111. Pappus and Desargues Theorem
172Pappus Theorem
C
- Let A, B, C be three collinear points and let A',
B' , C' be another triple of collinear points.
Let A'' be the intersection of (BC') and (B'C),
B'' the intersection (A,C') and (A'C), C'' the
intersetion of (AB') and (A'B). Then the points
A'', B'' and C'' are collinear.
B
A
C''
B''
A''
A'
B'
C'
173Desargues Theorem
B''
B'
- Let ABC and A'B'C' be two triangles. Let A'' be
the intersection of BC and B'C', let B'' be the
intersection of AC and A'C' and C'' be the
intersection of AB and A'B'. The lines AA',BB'
and CC' intersect in a common point O if and only
if A'', B'' and C'' are collinear.
A'
C''
A
O
B
C
C'
A''
174Ternary ring coordinatization.
b
- Ternary operation, desrcibed in geometric terms.
- Properties
- (a) x0b 0xbb
- (b)x10 1 x 0 x
- (c) Given x,y,a, there is a unique b such that y
xab - (d) Given x,x,y,y with x ¹ x there is a unique
ordered pair (a,b) such that y xab and
yxab. - (e) Given a,a,b,b with a ¹ a, there is a
unique x such that xabxab.
0,abc
b
0,c
1,b
0,b
0,0
1,0
a,0
175Pappian and Desarguesian Projective Planes
- Thm. A projective plane is desarguesian if and
only if the ternary ring is a field or a
sqew-field. - Thm. A projective plane is pappian if an only if
the ternary ring is a field.
176Non-Desarguesian Projective Plane
- F.R.Multon (1902)
- Points points in the real projective plane.
- Lines
- y