# The Max-Distance Network Creation Game on General Host Graphs

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1 The Max-Distance Network Creation Game on General Host Graphs 13 Luglio 2012

2 Introduction Network Creation Games are games that model the formation of large-scale networks governed by autonomous agents. E.g.: Social networks (friendship networks, collaboration networks, scientific citations,... ) Communication networks, e.g. the Internet Hosts are rational and egoistic agents who want to buy links in order to construct a good-quality network while spending the least possible. Not all links can be bought. An host can only buy a link (towards another host) if the link belongs to a given existing network infrastucture.

3 Problem definition Given an undirected and connected host graph H, and a parameter α 0, we define Maxgame+HG to be a strategic game V (H), Σ, C. The set of strategies of a player u V (H) is the power set of all the edges that are incident to u in H Σ u = P( {(u, v) E : v V } ) Given a strategy profile σ = σ u u V we define the graph G σ as: V (G σ ) = V ed E(G σ ) = u V Intuitively, G σ contains all the edges bought by the players. σ u

4 Problem definition The payoff of a player u w.r.t. σ is a cost composed by the sum of two quantities: The building cost: α times the number of edges bought by u. The usage cost: the eccentricity of the vertex u in G σ. Each player wants to minimize his cost, so he would like to pursue two conflicting objectives: Connect to few other hosts. Keep his eccentricy low. To summarize, the cost of the player u is: C u (σ) = α σ u + ε σ (u)

5 Problem definition A strategy profile σ is a Nash Equilibrium iff. every player cannot decrease his cost by changing his strategy (provided that the strategies of all the other players do not change). More precisely: Definition (Nash Equilibrium) A strategy profile σ is a Nash Equilibrium for Maxgame+HG if u V : C u (σ) C u ( σ u, σ u ) σ u Σ u

6 Problem definition Quite naturally, we can define a social cost function as the sum of the player s payoffs: SC(σ) = u V C u (σ) Let OPT be a strategy profile minimizing SC(σ). OPT is said to be a social optimum: OPT arg min SC(σ) σ

7 Problem definition Let σ ne a N.E., the quantity Q(σ) = quality. SC(σ) SC(OPT ) If Q(σ) is 1 then σ is a social optimum. is a measure of its If Q(σ) is large, then σ has a cost much bigger than a social optimum. The Price of Anarchy (PoA) [?] is the maximum of Q(σ) w.r.t. all the NEs: SC(σ) PoA = max σ : σ is a N.E. SC(OPT ) Intiutively, the PoA is an upper bound to the quality loss of an equilibrium caused by the egoistic behaviour of the players.

8 Example: Strategy profile α =

9 Example: Equilibrium α = Costo sociale: 21

10 Example: Social optimum α = Costo sociale: 20

11 Problems of interest for non-cooperative games Bound the quality loss of an equilibrium: Price of Anarchy. Computational issues. Finding equilibria. Computing the best response of a player w.r.t. the other strategies. Dynamics: Analisys of the best and better response dynamics. If the players play in turns, do they end up in a N.E.? Bounding the time needed for convergence.

12 Related works: Problems related to Maxgame+HG can be found in: [?] On a network creation game: First model for communication networks. Complete host graph. The usage cost is the routing cost (sum of the distances). [?] The price of anarchy in network creation games: Complete host graph. The usage cost is the eccenticity. [?] The price of anarchy in cooperative network creation games: Arbitrary host graph. The usage cost is the routing cost (sum of the distances). Maxgame+HG was not studied (at the time).

13 Why Maxgame+HG? Studying Maxgame+HG is interesting: It is a generalization of a prominent problem in the literature. Assuming the existence of a complete host graph is unrealistic. Insights on how the topology of the host graph affects the quality of the resulting networks.

14 Results (1/2) Results and bounds to the Price of Anarchy: Computing the best response of a player is NP Hard (reduction from Set-Cover) Maxgame+HG is not a potential game. Moreover the best response dynamics does not converge to an equilibrium if α > 0.

15 Results (2/2) Results and bounds to the Price of Anarchy: Notes Lower Bound Upper Bound ( ) max{ω n 1+α, n Ω(1+min{α, n α })} O( α+r H ) α n Ω(1) O(1) L equilibrio è un albero min{o(α + 1), O(r H )} E(H) = n 1 + k con O(k + 1) k = O(n) H è una griglia Ω(1 + min{α, n α }) O( n α+r H ) H è k-regolare con k 3 Ω(1 + min{α, n α }) O( n α+r H ) n = V (H), r H is the raius of H.

16 Potential game: definition A game is an (exact) potential game if it admits an (exact) potential function Φ on Σ = u V Σ u such that, when a player changes his strategy, the change is his payoff is equal to the change of the potential function. Definition ((Exact) Potential function) Φ : Σ R is an (exact) potential function if σ Σ, u V, σ u Σ u we have: C u ( σ u, σ u ) C u (σ) = Φ( σ u, σ u ) Φ(σ)

17 Potential game: properties A potential game has at least a N.E. The potential function Φ has a global minimum (it is defined on a finite domain). Every better response dynamics eventually converges to a N.E. (the value of Φ is monotonically decreasing). Theorem For every constant α Maxgame+HG is not a potential game. Moreover, if α > 0, the better response dynamics does not converge to a N.E.

18 Maxgame+HG is not a potential game Dimostrazione Suppose, by contraddiction, that Φ is a potential function for Maxgame+HG. For every value of α we will show an host graph an a sequence of strategy changes that will allow to arbitrarily decrease the value of Φ. The following liveness property will also hold: There exists a constant k > 0 such that, starting from any turn t, every player plays at least once between the turns (t + 1) and (t + k).

19 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 C(1) = α + l

20 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 C(1) = l 1 Player 1 saves α! 7 4 C(x) = α + l 6 5

21 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 C(x) = α + l 2 Player x saves 2! 7 4 C(1) = 2l 4 6 5

22 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 C(1) = α + l + 2 Player 1 saves l (α + 6) > Player 4 is in the same situation of player 1 at the beginning of the game.

23 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l x 4 3 We can repeat the previous strategy changes. The potential function must decrease by 2 each time. 6 5

24 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 First, the vertices 1, 4, 7,... l will play

25 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 Then, the vertices 3, 6, 9,..., l

26 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l 2 8 x 3 And finally, the vertices 2, 5, 8,... l

27 Maxgame+HG is not a potential game 9 l z 1 2 Constraints: l > α + 6 l 1 (mod 3) d(x, z) = l x 4 3 If we repeat the strategy changes one more time, we return to the starting configuration. 6 5 We have a strategy cycle!

28 Lower bounds to the PoA 0, 0 2, 0 4, 0 0, 0 0, 2 0, 4 0, 0 1, 1 1, 3 1, 5 2, 2 2, 4 2, 0 3, 1 3, 3 3, 5 4, 2 4, 4 4, 0 5, 1 5, 3 5, 5 0, 2 0, 4 0, 0 Definition (Quasi-torus H) V (H) has 2k 2 intersection vertices, i.e. the pairs (i, j) s.t. 0 i, j < 2k and i + j is even. Each vertex (i, j) V (H) has an edge towards {(i 1, j 1), (i 1, j + 1), (i + 1, j 1), (i + 1, j + 1)} modulo 2k. Each edge has weight l = 2(1 + α ).

29 Some properties Let X i,j be the set of vertices that are either on the i-th row or on the j-th column. The following properties hold: i) H is vertex-transitive. ii) The distance between (i, j) and (i, j ) is: l max { min { i i, 2k i i }, min { j j, 2k j j }} iii) The eccentricity of every vertex is lk. iv) 0 i, j 2k, the distance between a vertex v X i,j and i k, j k is lk. v) If (u, v) E(H) then the eccentricities of u and v in H l are at least l(k + 1)

30 Building the equilibrium and the Host Graph Let G be the unweigted graph obtained by replacing each edge (u, v) in H by a path of length l between u and v. The graph G has l 1 new vertices for each edge of H. The total number of vertices is therefore n = 2k 2 + 4k 2 (l 1) = Θ ( k 2 (1 + α) ) from which k = Θ ( n 1+α ). Let σ be a strategy profile such that G σ = G and intersection vertices haven t bought any edges. Let A be a set containing one vertex per row and one vertex per column. Let H be the graph obtained by adding to G all the vertices in X i,j i, j A. σ is an equilibrium for Maxgame+HG with host graph H.

31 Building the equilibrium and the Host Graph 0 2k 0 1 k 2k 0 k k, k 1 0

32 σ is a Nash Equilibrium Every intersection vertex i, j of G has eccentricity lk. By property (iv), i, j cannot improve its eccentricy using the edges towards X i,j. The vertices i, j haven t bought edges, so they cannot remove of swap edges. The vertices on the paths have an eccentricy of at most lk + l 2 as the nearest intersection vertex is at most at a distance of l 2. The vertices on the paths can only remove a single edge. By property (v), doing so would increase their eccentricity by at least l(k + 1) lk l 2 > α.

33 Bounding the PoA x u v y The radius of G is Ω(lk). The radius of H is O(l). A path between two vertices u and v can be built as the union of: 1 A subpath of length O(l) between u and an intersection vertex x. 2 A subpath of at most 4 edges in E(H) \ E(G) between x and an intersection vertex y near v. 3 A subpath of length O(l) between y and v.

34 Bounding the PoA x u v y PoA SC(σ) SC(OPT ) SC(G) SC(H) ( ) αn + nlk = Ω nl ( ) nlk = Ω = Ω(k) nl ( ) n = Ω 1 + α

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