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CanaDAM 2011. Almost all cop-win graphs contain a universal vertex. Anthony Bonato Ryerson University. Cop number of a graph. the cop number of a graph , written c(G) , is an elusive graph parameter few connections to other graph parameters hard to compute hard to find bounds

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CanaDAM 2011

Almost all cop-win graphs contain a universal vertex

Anthony Bonato

Ryerson University

cop number of a graph
Cop number of a graph
  • the cop number of a graph, written c(G), is an elusive graph parameter
    • few connections to other graph parameters
    • hard to compute
    • hard to find bounds
    • structure of k-cop-win graphs with k > 1 is not well understood

Random cop-win graphs Anthony Bonato

cops and robbers
Cops and Robbers
  • played on reflexive graphs G
  • two players CopsC and robber R play at alternate time-steps (cops first) with perfect information
  • players move to vertices along edges; allowed to moved to neighbors or pass
  • cops try to capture (i.e. land on) the robber, while robber tries to evade capture
  • minimum number of cops needed to capture the robber is the cop number c(G)
    • well-defined as c(G) ≤ γ(G)

Random cop-win graphs Anthony Bonato

fast facts about cop number
Fast facts about cop number
  • (Aigner, Fromme, 84) introduced parameter
    • G planar, then c(G) ≤ 3
  • (Berrarducci, Intrigila, 93), (Hahn, MacGillivray,06), (B, Chiniforooshan,10):

“c(G) ≤ s?” sfixed: running time O(n2s+3), n = |V(G)|

  • (Fomin, Golovach, Kratochvíl, Nisse, Suchan, 08): if s not fixed, then computing the cop number is NP-hard
  • (Shroeder,01) Ggenus g, then c(G)≤ ⌊ 3g/2 ⌋+3
  • (Joret, Kamiński, Theis, 09) c(G)≤ tw(G)/2

Random cop-win graphs Anthony Bonato

meyniel s conjecture
Meyniel’s Conjecture
  • c(n) = maximum cop number of a connected

graph of order n

  • Meyniel Conjecture: c(n) = O(n1/2).

Random cop-win graphs Anthony Bonato

state of the art
State-of-the-art
  • (Lu, Peng, 09+) proved that
  • independently proved by (Scott, Sudakov,10+), and (Frieze, Krivelevich, Loh, 10+)
  • even proving

c(n) = O(n1-ε)

for some ε > 0 is open

Random cop-win graphs Anthony Bonato

cop win case
Cop-win case
  • consider the case when one cop has a winning strategy
    • cop-win graphs
  • introduced by (Nowakowski, Winkler, 83), (Quilliot, 78)
    • cliques, universal vertices
    • trees
    • chordal graphs

Random cop-win graphs Anthony Bonato

characterization
Characterization
  • node u is a corner if there is a v such that N[v] contains N[u]
    • v is the parent; u is the child
  • a graph is dismantlable if we can iteratively delete corners until there is only one vertex

Theorem (Nowakowski, Winkler 83; Quilliot, 78)

A graph is cop-win if and only if it is dismantlable.

idea: cop-win graphs always have corners; retract corner

and play shadow strategy;

- dismantlable graphs are cop-win by induction

Random cop-win graphs Anthony Bonato

dismantlable graphs
Dismantlable graphs

Random cop-win graphs Anthony Bonato

dismantlable graphs1
Dismantlable graphs
  • unique corner!
  • part of an infinite family that maximizes capture time (Bonato, Hahn, Golovach, Kratochvíl,09)

Random cop-win graphs Anthony Bonato

cop win orderings
Cop-win orderings
  • a permutation v1, v2, … , vnof V(G) is a

cop-win ordering if there exist vertices w1, w2, …, wnsuch that for all i, wi is the parent of vi in the subgraph induced V(G) \ {vj : j < i}.

    • a cop-win ordering dismantlability

5

1

4

3

2

Random cop-win graphs Anthony Bonato

cop win strategy clarke nowakowski 2001
Cop-win Strategy (Clarke, Nowakowski, 2001)
  • V(G) = [n] a cop-win ordering
  • G1 = G, i > 1, Gi: subgraph induced by deleting 1, …, i-1
  • fi: Gi → Gi+1 retraction mapping i to a fixed one of its parents
  • Fi=fi-1 ○… ○ f2 ○ f1
    • a homomorphism
  • idea: robber on u, think of Fi(u) shadow of robber
    • cop moves to capture shadow
    • works as the Fi are homomorphisms
  • results in a capture in at most n moves of cop

Random cop-win graphs Anthony Bonato

random graphs g n p erd s r nyi 63
Random graphs G(n,p)(Erdős, Rényi, 63)
  • n a positive integer, p = p(n) a real number in (0,1)
  • G(n,p): probability space on graphs with nodes {1,…,n}, two nodes joined independently and with probability p

4

1

2

3

5

Random cop-win graphs Anthony Bonato

typical cop win graphs
Typical cop-win graphs
  • what is a random cop-win graph?
  • G(n,1/2) and condition on being cop-win
  • probability of choosing a cop-win graph on the uniform space of labeled graphs of ordered n

Random cop-win graphs Anthony Bonato

cop number of g n 1 2
Cop number of G(n,1/2)
  • (B,Hahn, Wang, 07), (B,Prałat, Wang,09)

A.a.s.

c(G(n,1/2)) = (1+o(1))log2n.

-matches the domination number

Random cop-win graphs Anthony Bonato

universal vertices
Universal vertices
  • P(cop-win) ≥ P(universal)

=n2-n+1 – O(n22-2n+3)

= (1+o(1))n2-n+1

  • …this is in fact the correct answer!

Random cop-win graphs Anthony Bonato

main result
Main result

Theorem (B,Kemkes, Prałat,11+)

In G(n,1/2),

P(cop-win) = (1+o(1))n2-n+1

Random cop-win graphs Anthony Bonato

corollaries
Corollaries

Corollary (BKP,11+)

The number of labeled cop-win graphs is

Random cop-win graphs Anthony Bonato

corollaries1
Corollaries

Un= number of labeled graphs with a universal

vertex

Cn= number of labeled cop-win graphs

Corollary (BKP,11+)

That is, almost all cop-win graphs contain a

universal vertex.

Random cop-win graphs Anthony Bonato

strategy of proof
Strategy of proof
  • probability of being cop-win and not having a universal vertex is very small
  • P(cop-win + ∆ ≤ n – 3) ≤ 2-(1+ε)n
  • P(cop-win + ∆ = n – 2) = 2-(3-log23)n+o(n)

Random cop-win graphs Anthony Bonato

p cop win n 3 2 1 n
P(cop-win + ∆ ≤ n – 3) ≤2-(1+ε)n
  • consider cases based on number of parents:
    • there is a cop-win ordering whose vertices in their initial segments of length 0.05n have more than 17 parents.
    • there is a cop-win ordering whose vertices in their initial segments of length 0.05n have at most 17 parents, each of which has co-degree more than n2/3.
    • there is a cop-win ordering whose initial segments of length 0.05n have between 2 and 17 parents, and at least one parent has co-degree at most n2/3.
    • there exists a vertex w with co-degree between 2 and n2/3, such that wi = w for i ≤ 0.05n.

Random cop-win graphs Anthony Bonato

p cop win n 2 2 3 log 2 3 n o n
P(cop-win + ∆ = n – 2) ≤2-(3-log23)n+o(n)

Sketch of proof: Using (1), we obtain that there is an ε > 0

such that

P(cop-win) ≤P(cop-win and ∆ ≤ n-3) + P(∆ ≥ n-2)

≤ 2-(1+ε)n + n22-n+1

≤ 2-n+o(n) (*)

  • if ∆ = n-2, then G has a vertex w of degree n-2, a unique vertex v not adjacent to w.
    • let A be the vertices not adjacent to v (and adjacent to w)
    • let B be the vertices adjacent to v (and also to w)
  • Claim: The subgraph induced by B is cop-win.

Random cop-win graphs Anthony Bonato

slide23

w

A

B

x

v

Random cop-win graphs Anthony Bonato

proof continued
Proof continued
  • n choices for w; n-1 for v
  • choices for A
  • if |A| = i, then using (*), probability that Bis cop-win is at most 2-n+2+i+o(n)

Random cop-win graphs Anthony Bonato

problems
Problems
  • do almost all k-cop-win graphs contain a dominating set of order k?
    • would imply that the number of labeled k-cop-win graphs of order n is
    • difficulty: no simple elimination ordering for k > 1 (Clarke, MacGillivray,09+)
  • characterizing cop-win planar graphs
  • (Clarke, Fitzpatrick, Hill, Nowakowski,10): classify the cop-win graphs which have cop number 2 after a vertex is deleted

Random cop-win graphs Anthony Bonato

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preprints, reprints, contact:

Google: “Anthony Bonato”

Random cop-win graphs Anthony Bonato

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