Lecture 4: Mathematics of Networks (Cont)

1. Lecture 4: Mathematics of Networks (Cont) CS 765: Complex Networks Slides are modified from Networks: Theory and Application by Lada Adamic

2. Trees • Trees are undirected graphs that contain no cycles • For n nodes, number of edges m = n-1 • A node can be dedicated as the root

3. examples of trees • In nature • trees • river networks • arteries (or veins, but not both) • Man made • sewer system • Computer science • binary search trees • decision trees (AI) • Network analysis • minimum spanning trees • from one node – how to reach all other nodes most quickly • may not be unique, because shortest paths are not always unique • depends on weight of edges

4. Planar graphs • A graph is planar if it can be drawn on a plane without any edges crossing

5. Cliques and complete graphs • Kn is the complete graph (clique) with K vertices • each vertex is connected to every other vertex • there are n*(n-1)/2 undirected edges K5 K3 K8

6. Kuratowski’s theorem • Every non-planar network contains at least one subgraph that is an expansion of K5 or K3,3. K5 K3,3 Expansion: Addition of new node in the middle of edges. • Research challenge: Degree of planarity?

7. #s of planar graphs of different sizes 1:1 2:2 3:4 4:11 Every planar graph has a straight line embedding

8. Edge contractions defined • A finite graph G is planar if and only if it has no subgraph that is homeomorphic or edge-contractible to the complete graph in five vertices (K5) or the complete bipartite graph K3, 3. (Kuratowski's Theorem)

9. Peterson graph • Example of using edge contractions to show a graph is not planar

10. Bi-cliques (cliques in bipartite graphs) • Km,n is the complete bipartite graph with m and n vertices of the two different types • K3,3 maps to the utility graph • Is there a way to connect three utilities, e.g. gas, water, electricity to three houses without having any of the pipes cross? Utility graph K3,3

11. 2 Node degree 3 1 • Outdegree = 4 5 A = example: outdegree for node 3 is 2, which we obtain by summing the number of non-zero entries in the 3rdrow • Indegree = A = example: the indegree for node 3 is 1, which we obtain by summing the number of non-zero entries in the 3rdcolumn

12. Degree sequence and Degree distribution • Degree sequence: An ordered list of the (in,out) degree of each node • In-degree sequence: • [2, 2, 2, 1, 1, 1, 1, 0] • Out-degree sequence: • [2, 2, 2, 2, 1, 1, 1, 0] • (undirected) degree sequence: • [3, 3, 3, 2, 2, 1, 1, 1] • Degree distribution: A frequency count of the occurrence of each degree • In-degree distribution: • [(2,3) (1,4) (0,1)] • Out-degree distribution: • [(2,4) (1,3) (0,1)] • (undirected) distribution: • [(3,3) (2,2) (1,3)]

13. Structural Metrics: Degree distribution What if it is directed ?

14. Characterizing networks:How dense are they?

15. network metrics: graph density • Of the connections that may exist between n nodes • directed graph emax = n*(n-1) • undirected graphemax = n*(n-1)/2 • What fraction are present? • density = e/ emax • For example, out of 12 possible connections, this graph has 7, giving it a density of 7/12 = 0.583

16. Graph density • Would this measure be useful for comparing networks of different sizes (different numbers of nodes)? • As n → ∞, a graph whose density reaches • 0 is a sparse graph • a constant is a dense graph

17. Characterizing networks:How far apart are things?

18. Network metrics: paths • A path is any sequence of vertices such that every consecutive pair of vertices in the sequence is connected by an edge in the network. • For directed: traversed in the correct direction for the edges. • path can visit itself (vertex or edge) more than once • Self-avoiding paths do not intersect themselves. • Path length r is the number of edges on the path • Called hops

19. Network metrics: paths

20. Network metrics: shortest paths B 3 C A 2 1 3 D E 2

21. Structural metrics: Average path length 1 ≤ L ≤ D ≤ N-1

22. Eulerian Path • Euler’s Seven Bridges of Königsberg • one of the first problems in graph theory • Is there a route that crosses each bridge only once and returns to the starting point? Source: http://en.wikipedia.org/wiki/Seven_Bridges_of_Königsberg Image 1 – GNU v1.2: Bogdan, Wikipedia; http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License Image 2 –GNU v1.2: Booyabazooka, Wikipedia; http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License Image 3 –GNU v1.2: Riojajar, Wikipedia; http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License

23. Eulerian and Hamiltonian paths • Hamiltonian path is self avoiding If starting point and end point are the same: only possible if no nodes have an odd degree as each path must visit and leave each shore If don’t need to return to starting point can have 0 or 2 nodes with an odd degree Eulerian path: traverse each edge exactly once Hamiltonian path: visit each vertex exactly once

24. Characterizing networks:Is everything connected?

25. Network metrics: components • If there is a path from every vertex in a network to every other, the network is connected • otherwise, it is disconnected • Component: A subset of vertices such that there exist at least one path from each member of the subset to others and there does not exist another vertex in the network which is connected to any vertex in the subset • Maximal subset • A singeleton vertex that is not connected to any other forms a size one component • Every vertex belongs to exactly one component

26. components in directed networks • Weakly connected components: every node can be reached from every other node by following links in either direction • Weakly connected components • A B C D E • G H F • Strongly connected components • Each node within the component can be reached from every other node in the component by following directed links B B F F G • Strongly connected components • B C D E • A • G H • F C G A C A H D H E D E

27. components in directed networks • Every strongly connected component of more than one vertex has at least one cycle • Out-component: set of all vertices that are reachable via directed paths starting at a specific vertex v • Out-components of all members of a strongly connected component are identical • In-component: set of all vertices from which there is a direct path to a vertex v • In-components of all members of a strongly connected component are identical B F G C A H D E

28. network metrics: size of giant component • if the largest component encompasses a significant fraction of the graph, it is called the giant component

29. bowtie model of the web • The Web is a directed graph: • webpages link to other webpages • The connected components tell us what set of pages can be reached from any other just by surfing • no ‘jumping’ around by typing in a URL or using a search engine • Broder et al. 1999 – crawl of over 200 million pages and 1.5 billion links. • SCC – 27.5% • IN and OUT – 21.5% • Tendrils and tubes – 21.5% • Disconnected – 8%

30. Independent paths • Edge independent paths: if they share no common edge • Vertex independent paths: if they share no common vertex except start and end vertices • Vertex-independent => Edge-independent • Also called disjoint paths • These set of paths are not necessarily unique • Connectivity of vertices: the maximal number of independent paths between a pair of vertices • Used to identify bottlenecks and resiliency to failures

31. Cut Sets and Maximum Flow • A minimum cut set is the smallest cut set that will disconnect a specified pair of vertices • Need not to be unique • Menger’s theorem: If there is no cut set of size less than n between a pair of vertices, then there are at least n independent paths between the same vertices. • Implies that the size of min cut set is equal to maximum number of independent paths (for both edge and vertex independence) • Maximum Flow between a pair of vertices is the number of edge independent paths times the edge capacity.

32. Graph Laplacian