Protecting a Graph

1. Protecting a Graph Chip Klostermeyer

2. 6 vertices 7 edges Graph(a.k.a. Network)

4. 6 vertices 7 edges Dominating Set γ=2 Graph

5. 6 vertices 7 edges Independent Set β=3 Graph

6. Clique with 4 vertices in a graph

7. 6 vertices 10 edges Clique Cover Θ=2 Graph

8. Eternal Dominating Set • Defend graph against sequence of attacks at vertices • At most one guard per vertex • Send guard to attacked vertex • Guards must induce dominating set • One guard moves at a time (later, we allow all guards to move)

9. 2-player game • Attacker chooses vertex with no guard to attack • Defender chooses guard to send to attacked vertex (must be sent from neighboring vertex) • Attacker wins if after some # of attacks, guards do not induce dominating set • Defender wins otherwise

10. Eternal Dominating Set γ∞=3 γ=2 Attacked Vertex in red Guards on black vertices

11. ? ? Eternal Dominating Set γ∞=3 γ=2 Second attack at red vertex forces guards to not be a dominating set. 3 guards needed

12. Eternal Dominating Set γ∞=3 γ=2 3 guards needed

13. Applications Military Defense (original problem dates to Emperor Constantine) Autonomous Systems (foolproof model) File Migration File Migration for server maintenance (eviction model...Netflix chaos monkey)

14. Complexity • Question of deciding if a set D is an eternal dominating set is hard for co-NPNP But what class is it in? (Is in PSPACE) Problem is NP-complete if set of requests finite and given as part of input. Polynomial time for trees, for example

15. Basic Bounds γ≤β≤γ∞ ≤ Θ Because one guard can defend a clique and attacks on an independent set of size k require k different guards

16. Problem Goddard, Hedetniemi, Hedetniemi asked if γ∞ ≤ c * β And they showed graphs for which γ∞ < Θ Smallest known has 11 vertices …

17. Upper Bound Klostermeyer and MacGillivray proved γ∞ ≤ C(β+1, 2) C(n, 2) denotes binomial coefficient Proof is algorithmic.

18. Proof idea Guards located on independent sets of size 1, 2, …,β Defend with guard from smallest set possible

19. Proof idea Guards located on independent sets of size 1, 2, …,β Swapping guard with attacked vertex destroys independence!! Solution….

20. Proof idea Guards located on independent sets of size 1, 2, …,β Choose union of independent sets to be LARGE as possible

21. Proof idea Guards located on independent sets of size 1, 2, …,β After yellow guard moves, we have all our independent sets.

22. Key points in proof • Independent sets induce a dominating set since independent set of size β is a dominating set. • Can show that even if guard moves from the independent set of size β, after move there will still be an independent set of size β.

23. Lower Bound? • Upper bound: γ∞ ≤ C(β+1, 2) • But is it tight? • Yes. Goldwasser and Klostermeyer proved that certain (large) complements of Kneser graphs require this many guards.

24. γ≤β≤γ∞ ≤ Θ γ∞=Θ for Perfect graphs [follows from PGT] Series-parallel graphs [Anderson et al.] Powers of Cycles and their complements [KM] Circular-arc graphs [Regan] Open problem: planar graphs

25. Open Questions Is there a graph G with γ = γ∞ < Θ ? No triangle-free; none with maximum-degree three. Is there a triangle-free graph G with β = γ∞ < Θ ? Is γ∞(G x H) ≥ γ∞ (G) γ∞ (H)?

26. M-Eternal Dominating Set γ∞m=2 All guards can move in response to attack

27. M-Eternal Dominating Sets γ≤γ∞m ≤ β Exact bounds known for trees, 2 by n, 4 by n grids 3 by n grids: ≤ 8n/9 guards needed 2 by 3 grid: 2 guards suffice Conjecture: # guards needed in n by n grid is γ + O(1)

28. 7 vertices 8 edges Total Dominating Set γt=3 Total Domination in Graphs

29. Eternal Total Domination • Require dominating set to be total at all times. • Example: 4 guards (if one moves at a time). 3 guards (if all can move) Guards move up and down in tandem

30. Eternal Total Domination γ∞<γ∞t ≤ γ∞ + γ≤ 2Θ γ≤γt ≤ γ∞tm ≤ 2Θ-1 We characterize the graphs where the last inequality is tight. Exact bounds known for 2 by n and 3 by n grids.

31. Protecting Edges • Attacks on edges: guard must cross attacked edge. All guards move. • Guards must induce a VERTEX COVER α = 3

32. Protecting Edges α∞ = 3

33. Results • α≤α∞≤ 2α • Graphs achieving upper bound characterized [Klost.-Mynhardt] • Trees require # internal vertices + 1

34. Edge Protection • Which graphs have α = α∞? • Grids • Kn X G • Circulants, others. Is it true for vertex-transitive graphs? Is it true for G X H if it is true for G and/or H?

35. More Edge Protection • Which graphs have α∞ =γ∞m ?? • We characterize which trees. • No bipartite graph with δ ≥ 2 except C4 • No graph with δ ≥ 2 except C4 • Graphs with pendant vertices?? Explain criticality in edge protection!

36. Vertex Cover • m-eternal domination number is less than eternal vertex cover number for all graphs of minimum degree 2, except for C4. • m-eternal domination number is less than vertex cover number for all graphs of minimum degree 2 and girth 7 and ≥ 9. • What about 5, 6, 8?

37. Eviction Model: One Guard Moves e∞=2 γ=2 Attacked Vertex in red Attacked guard must have empty neighbor

38. Eviction: One guard moves • e∞ ≤ Θ • e∞ ≤ β for bipartite graphs • e∞ > β for some graphs • e∞ ≤ β when β=2 • e∞ ≤ 5 when β = 3 • Question: is e∞ ≤ γ∞ for all G?

39. Eviction Model: All Guards Move e∞m = 2 Attacked vertex must remain empty for one time period

40. Eviction: All guards move • em∞ ≤ β • Question: Isem∞ ≤ γ∞m for all G?

41. Thank you!