CSE 522 – Algorithmic and Economic Aspects of the Internet

1. CSE 522 – Algorithmic and Economic Aspects of the Internet Instructors: Nicole Immorlica Mohammad Mahdian

2. Today More about advertisement auctions Some open questions

3. Generative models • Have Seen • Probabilistic models for power law graphs and small world networks • Network formation games • Open Questions • More realistic models for the Internet graph, hierarchical (i.e. hosts and pages) • Combining ideas from both probabilistic models and game theoretic models

4. Search in Social Networks • Have Seen • Kleinberg’s decentralized search • Generalizations to arbitrary geographic distributions • Open Questions • New Kleinberg paper, FOCS 2005, discusses payment for search in social networks • Generalizations: general graphs, cost for forwarding information, response times

5. Link Analysis Algorithms • Have Seen • Link analysis for web graph – PageRank, HITS • Web spam • Axiomatic approaches to PageRank • Open questions • Collusion in PageRank: Algorithms for boosting PageRank of a particular node that locally look “normal”

6. Weblogs • A social network • Time-stamped links • Frequent updates • RSS feed • Open Questions • Generative models • Finding “discussions” • Blogs to predict trends (sales, etc.), finding correlated topics (also in query logs) 1 2 1 2 3 4 4 3 Picture from Kumar et al.

7. Query Logs Income Tax

8. Query Logs Greeting Cards

9. Query Log Analysis • Correlation between terms

10. Query Log Analysis • Data streams (top gainers & top losers) • Two streams S1 and S2 of items from universe U={1,…,n} S1: 1 1 1 2 5 4 2 4 2 2 1 2 1 S2: 1 5 5 1 4 1 2 5 4 1 5 1 3 • Find item i with approximately largest (or smallest) count(i,S1)-count(i,S2) • One pass, sublinear space • Charikar et al. give 2-pass algorithm

11. Clustering and Communities • Have Seen • Spectral clustering (use eigenvectors of adjacency matrix to well-connected components) • Correlation clustering (min cut positive edges and uncut negative edges) • Metric labeling (assign labels to vertices to min assignment plus separation cost) • Open Questions • Combine correlation clustering and metric labeling • Clustering in directed graphs • Finding dense bipartite subgraphs in the web (Kumar et al. have heuristic for complete subgraphs which finds small communities)

12. Reputation Systems • Have Seen • Equilibrium analysis of a model with one seller and multiple buyers • Open Questions • Better game-theoretic analysis of existing systems (multiple sellers, identity fraud, buyer reputation) • Better methods for aggregating feedbacks (taking into account value of transactions; reputation of the person leaving the feedback)

13. Peer-to-peer networks • Have Seen • Models (Napster, Gnutella, Chord) • Content storage, search, and dynamics • Open Questions • Spread of viruses, honeypots • Incentives for providing content • Self-stabilization

14. Recommendation Systems • Have Seen • History-based systems • Two models, used assumptions about clustering of topics, small number of clusters, existence of orthogonal dominant user types • Query-based systems • Two models, relied on “committees”, also assumed existence of small number of user types • Open Questions • Economic incentives for recommendations

15. The End!