Graphs represented by words
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Transcript of Graphs represented by words
Graphs represented by words Sergey Kitaev
Reykjavik University
Sobolev Institute of Mathematics
Joint work with
Artem Pyatkin
Magnus M. Halldorsson
Reykjavik University
Basic definitions
Sergey Kitaev Graphs represented by words
A finite word over {x,y} is alternating if it does not contain xx and yy.
Alternating words: yx, xy, xyxyxyxy, yxy, etc.
Non-alternating words: yyx, xyy, yxxyxyxx, etc.
Letters x and y alternate in a word w if they induce an alternating subword.
x and y alternate in w = xyzazxayxzyax
Basic definitions
Sergey Kitaev Graphs represented by words
A finite word over {x,y} is alternating if it does not contain xx and yy.
Alternating words: yx, xy, xyxyxyxy, yxy, etc.
Non-alternating words: yyx, xyy, yxxyxyxx, etc.
Letters x and y alternate in a word w if they induce an alternating subword.
x and y alternate in w = xyzazxayxzyax
x and y do not alternate in w = xyzazyaxyxzyax
Basic definitions
Sergey Kitaev Graphs represented by words
A word w is k-uniform if each of its letters appears in w exactly k times.
A 1-uniform word is also called a permutation.
A graph G=(V,E) is represented by a word w if 1. Var(w)=V, and2. (x,y) E iff x and y alternate in w.
word-representant
A graph is (k-)representable if it can be represented by a (k-uniform) word.
A graph G is 1-representable iff G is a complete graph.
Example of a representable graph
Sergey Kitaev Graphs represented by words
cycle graph
x
y
v
z a
xyzxazvay represents the graph
xyzxazvayv 2-represents the graph
Switching the indicated x and a would create an extra edge
Sergey Kitaev Graphs represented by words
Cliques and Independent Sets
Kn
Clique
Kn
Independent set
W=ABC...Z ABC...Z W=ABC...YZ ZY...CBA
V={A,B,C,...Z}
Sergey Kitaev Graphs represented by words
Original motivation to study such representable graphs: The Perkins semigroup
S. Kitaev, S. Seif: Word problem of the Perkins semigroup via directed acyclic graphs, Order (2009).
Related work: Split-pair arrangement (application:
scheduling robots on a path, periodically)R. Graham, N. Zhang: Enumerating split-pair arrangements, J. Combin. Theory A, Feb. 2008.
Sergey Kitaev Graphs represented by words
Papers on representable graphs:
S. Kitaev, A. Pyatkin: On representable graphs, Automata, Languages and Combinatorics (2008).
M. Halldorsson, S. Kitaev, A. Pyatkin: On representable graphs, semi-transitive orientations, and the representation numbers, preprint.
Operations Preserving Representability
• Replacing a node v by a module H– H can be any clique or any comparability graph– Neighbors of v become neighbors of all nodes in H
• Gluing two representable graphs at 1 node
• Joining two representable graphs by an edge
Sergey Kitaev Graphs represented by words
G H+ = H G
G H& = G H
Operations Not Preserving Representability
• Taking the line graph
• Taking the complement
• Attaching two graphs at more than 1 node
Sergey Kitaev Graphs represented by words
G H+ = H G
Open question: Does it preserve non-representability?
The graph in red is not 2- or 3-representable. It is not known if it is representable or not.
Properties of representable graphs
Sergey Kitaev Graphs represented by words
If G is k-representable and m>k then G is m-representable.
For representable graphs, we may restrict ourselves to connected graphs.
G U H (G and H are two connected components) is representable iffG and H are representable. (Take concatenation of the corresponding words representants having at least two copies of each letter.)
If G is representable then G is k-representable for some k.
2-representable graphs
• 1-representable graphs cliques• 2-representable graphs ??
• A B C D E F G H C D H G F A B D
Sergey Kitaev Graphs represented by words
2-representable graphs
• View as overlapping intervals: u & v adjacent if they overlap
Example:
A B C D E F G H C D H G F A B E
Sergey Kitaev Graphs represented by words
E
A
F
u vuv E
2-representable graphs
• View as overlapping intervals:
Equivalent to Interval overlap graphs
A B C D E F G H C D H G F A B E
Sergey Kitaev Graphs represented by words
E
A
F
2-representable graphs
Sergey Kitaev Graphs represented by words
2-representable graphs
Sergey Kitaev Graphs represented by words
Circle graphs
Comparability graphs
• We can orient the edges to form a transitive digraph
• They correspond to partial orders.
Sergey Kitaev Graphs represented by words
Comparability graphs
• We can orient the edges to form a transitive digraph
• They correspond to partial orders.
Sergey Kitaev Graphs represented by words
Comparability graphs
• We can orient the edges to form a transitive digraph
• They correspond to partial orders.
Sergey Kitaev Graphs represented by words
Representing comparability graphs
1. Form a topological ordering, where a given letter, say c, is as early as possible: abcdefg
Sergey Kitaev Graphs represented by words
e
b
c
a
g
fd
Representing comparability graphs
1. Form a topological ordering, where a given letter, say c, is as early as possible: abcdefg
2. Then add another where it is as late as possible abfgdce
3. Repeat from 1. until done
Sergey Kitaev Graphs represented by words
e
b
c
a
g
fd
Representing comparability graphs
1. The resulting substring abcdefg abfgdcecovers all non-edges incident on c.
Sergey Kitaev Graphs represented by words
e
b
c
a
g
fd
Representing comparability graphs
1. The resulting substring abcdefg abfgdcecovers all non-edges incident on c.
2. For this graph it would suffice to repeat this for f: abfgcde abcdefgplus one round for d: dabcdfg
3. Final string:
Sergey Kitaev Graphs represented by words
e
b
c
a
g
fd
abcdefg abfgdce abfgcde abcdefg dabcdfg
Properties of representable graphs
Sergey Kitaev Graphs represented by words
A graph is permutationally representable if it can be represented by a word of the form P1P2...Pk where Pis are permutations of the same set.
1
2
3
4
is permutationally representable (13243142)
Lemma (Kitaev and Seif). A graph is permutationally representable iff it is transitively orientable, i.e. if it is a comparability graph.
Shortcut – a type of digraph
• Acyclic, non-transitive• Contains directed cycle
a, b, c, d, except last edge is reversed
• Non-transitive Not representable
Sergey Kitaev Graphs represented by words
d
b
c
a Missing!
Main result
• A graph G is representable iff G is orientable to a shortcut-free digraph
• () Straightforward. • () We give an algorithm that takes any shortcut-
free digraph and produces a word that represents the graph
Sergey Kitaev Graphs represented by words
Sketch of our algorithm
• Chain together copies of the digraph (= D’)– If ab D, then biai+1 D’
Sergey Kitaev Graphs represented by words
b c
d
a
Sketch of our algorithm
• Chain together copies of the digraph (= D’)– If ab D, then biai+1 D’
Sergey Kitaev Graphs represented by words
b c
d
a
b c
d
a
Sketch of our algorithm
• Chain together copies of the digraph (= D’)– If ab D, then biai+1 D’
• Form a topsort of D’ of pairs of copies.– In 1st copy, some letter d occurs as
late as possible– In 2nd copy d occurs as early as
possible
Sergey Kitaev Graphs represented by words
b c
d
a
b c
d
a
a b c a d c b dExample:
We allow the topsort to traverse the 2nd copy before finishing the 1st . The added edges ensure that adjacent nodes still alternate.
Size of the representation
• The algorithm creates a word where each of the n letters appears at most n times.
Each representable graph is n-representable• There are graphs that require n/2 occurrences
– E.g. based on the cocktail party graph
• Deciding whether a given graph is k-representable, for k between 3 and [n/2], is NP-complete
Sergey Kitaev Graphs represented by words
Corollary: 3-colorable graphs
• 3-colorable graphs are representable
• Red->Green->Blue orientation is shortcut-free!Sergey Kitaev Graphs represented by words
Non-representable graphs
Sergey Kitaev Graphs represented by words
Lemma. Let x be a vertex of degree n-1 in G having n nodes. Let H=G \ {x}. Then G is representable iff H is permutationally representable.
The lemmas give us a method to construct non-representable graphs.
Construction of non-representable graphs
Sergey Kitaev Graphs represented by words
1. Take a graph that is not a comparability graph (C5 is the smallest example);
2. Add a vertex adjacent to every node of the graph;3. Add other vertices and edges incident to them (optional).
W5 – the smallest non-representable graph
All odd wheels W2t+1 for t ≥ 2are non-representable graphs.
Small non-representable graphs
Sergey Kitaev Graphs represented by words
Relationships of graph classes
Sergey Kitaev Graphs represented by words
Representable
Circle 2-repres.
3-colorable Comparability
Bipartite2-inductive
Partial 2-trees
Outerplanar
2-outerplanar
3-trees
Trees
Chordal
2-trees
Perfect
Planar
4-colorable & K4-free
Split
Sergey Kitaev Graphs represented by words
A property of representable graphs
G representable For each x V, G[N(x)] is permutationally representable,
Natural question: Is the converse statement true?
G[N(x)] = graph induced by the neighborhood of x
Main means of showing non-representability
A non-representable graph whose induced neighborhood graphs are all comparability
Sergey Kitaev Graphs represented by words
co-T2 T2
3-representable graphs
Sergey Kitaev Graphs represented by words
examples of prisms
Theorem (Kitaev, Pyatkin). Every prism is 3-representable.
Theorem (Kitaev, Pyatkin). For every graph G there exists a 3-representable graph H that contains G as a minor.
In particular, a 3-subdivision of every graph G is 3-representable.
Sergey Kitaev Graphs represented by words
One more result
We can construct graphs with represntation number k=[n/2]
Coctail party graph:
Sergey Kitaev Graphs represented by words
One more result
We can construct graphs with represntation number k=[n/2]
Coctail party graph:
Complexity
• Recognizing representable graphs is in NP– Certificate is an orientation– Is it NP-hard?
• Most optimization problems are hard– Ind. Set, Dom. Set, Coloring, Clique Partition...
• Max Clique is polynomially solvable on repr.gr.– A clique is contained within some neighborhood– Neighborhoods induce comparability graphs
Sergey Kitaev Graphs represented by words
• Is it NP-hard to decide whether a given graph is representable?
• What is the maximum representation number of a graph (between n/2 and n)?
• Can we characterize the forbidden subgraphs of representative graphs?
• Graphs of maximum degree 4? • How many (k-)representable graphs are there?
Sergey Kitaev Graphs represented by words
Open problems
Sergey Kitaev Graphs represented by words
Resolved question
Is the Petersen’s graph representable?
Sergey Kitaev Graphs represented by words
Resolved question
Is the Petersen’s graph representable?
It is 3-representable:
1
2
34
9 8
7
6
5 10
1,3,8,7,2,9,6,10,7,4,9,3,5,4,1,2,8,3,10,7,6,8,5,10,1,9,4,5,6,2
Sergey Kitaev Graphs represented by words
Resolved questions
Are there any non-representable graphs with N(v) inducing a comparability graphs for every vertex v? In particular,– Are there non-representable graphs of maximum
degree 3?– Are there 3-chromatic non-representable graphs?– Are there any triangle-free non-representable
graphs?
Sergey Kitaev Graphs represented by words
Open/Resolved problems
• Is it NP-hard to determine whether a given graph is NP-representable.
• Is it true that every representable graph is k-representable for some k?
• How many (k-)representable graphs on n vertices are there?
Sergey Kitaev Graphs represented by words
Thank you for your attention!
THE END