Strategic Network Formation With Structural Holes
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Transcript of Strategic Network Formation With Structural Holes
Strategic Network Formation With Structural Holes
By Jon Kleinberg, Siddharth Suri, Eva Tardos, Tom Wexler
Structural Holes
Structural holes theory suggests that node A is in a stronger position than the other nodes, because it can control the flow of information between the three otherwise independent groups of nodes
Structural Holes
The paper looks at what would happen to a social network graph if all the nodes were incentivized to become 'bridging' nodes
The Model
The payoff for a node u is
a |N(u)| + Σv,w N∈ (u) (β(rvw ) ) −Σv L(u)∈ (cuv ), where
a = the static benefit associated with having a link with another node
N(u) = the number of nodes connected to u
β = any decreasing function
rvw = the number of length 2 paths between v and w, if v and w are not connected, 0 otherwise
L(u) = the number of nodes u has bought a link to
cuv = the cost associated with the u,v edge
Computing a node's best move
Can be done in polynomial time
Proof in the paper, via a reduction to the largest weight ideal problem, which can be reduced to the minimum cut of a network
What kinds of graphs does this create?
Does equilibria exist for any number of nodes?
Can we always reach equilibria using best response updates?
Experiments: The possibility of cycling
a = .9β(r) = 2a/rcxy = 1
The Cost Matrix: Uniform
We first look at what would happen if the 'cost' of maintaining an edge was constant (in this case, cuv = 1 for every edge), and will try to answer the following questions:
Does there always exist some equilibrium, for a graph of n nodes?
If so, is it always reachable by round robin best response updates?
Does equilibrium exist: Uniform Metric
Let Gn,k be a multipartite graph of n nodes, where the nodes are split up into n/k roughly equal sized groups, and every node in the ith group buys connections to every node in the jth group, for all j<i
Does equilibrium exist: Uniform Metric
Can we chose k such that Gn,k is at equilibrium?
Yes - we do this by defining a benefit function B(n,k) = k(a-1) + Ck,2 β(n-k), and picking k' such that B(n, k')>0 and B(n, k'-1)<=0
Can we always reach equilibrium: Uniform Metric
We have shown that for any n, there is always a k, such that Gn,k is in equilibrium.
Will our algorithm for computing best response dynamics reach an equilibrium?
Do other Equilibria exist: Uniform
Yes! After running several
experiments, all equilibria were found to be dense, Ω(n^2) edges
The paper then proves that all equilibria are dense, assuming rβ(r) >0
The Cost Matrix: Hierarchical
Useful for situations like the dynamics of a large company's social network
Here, we let the cost cuv, be the unique simple path between nodes u,v in the tree
Does equilibrium exist: Hierarchical Metric
This is still an open question, for arbitrarily large n
However, running experiments suggest that when equilibrium does exist, it occurs with a small group of people with links to everyone, a few people with a significant number of links, and most with very few links
Average degree being O(√n)
Conclusions
In both hierarchical and uniform metrics, we end up with a network divided into social classes, where a small number of nodes maintain O(n) links, and most nodes have much less
Even starting from an empty graph, the bridging incentive causes a break in the symmetry, but what happens under different bridging conditions?
Other Research Sanjeev Goyal and Fernando Vega-Redondo's
'Structural holes in social networks' uses a model where a node u receives benefits from residing on arbitrarily long paths between two other nodes, w and v. Here, star networks turn out to be the most robust equilibrium, for a wide range of parameters
Vincent Buskens and Arnout van de Rijt's 'Dynamics of networks if everyone strives for structural holes' looks at only benefits from length 2 paths, but uses a stricter form of equilibrium, which they call unilateral stability
Questions?