## First Principles and Effective Theory Approaches in Complex Networks

**Speaker:**Nima Dehmamy

**When:**November 2, 2015 (Mon), 01:00PM to 02:00PM (add to my calendar)

**Location:**SCI 328

*This event is part of the PhD Final Oral Exams. *

Examining Committee: H.Eugene Stanley, Pankaj Mehta, Shlomo Havlin, Kevin Black, Irena Vodenska

Abstract: This dissertation concerns modeling two aspects of dynamics of complex networks: (1) response dynamics and (2) growth and formation. A particularly challenging class of networks are ones in which both nodes and links are evolving over time {the most prominent example is a financial network. In the first part of the dissertation we present a model for the response dynamics in networks near a meta- stable point. We start with a Landau-Ginzburg approach and show that the most general lowest order Lagrangians for dynamical weighted networks can be used to derive conditions for stability under external shocks. Using a closely related model, which is easier to solve numerically, we propose a powerful and intuitive set of equations for response dynamics of financial networks. We find the stability conditions of the model and find two phases: "calm" phase, in which changes are sub-exponential and where the system moves to a new, close-by equilibrium; "frantic" phase, where changes are exponential, with negative blows resulting in crashes and positive ones leading to formation of "bubbles". We empirically verify these claims by analyzing data from Eurozone crisis of 2009-2012 and stock markets. We show that the model correctly identifies the time-line of the Eurozone crisis, and in the stock market data it correctly reproduces the auto-correlations and phases observed in the data. The second half of the dissertation addresses the following question: Do networks that form due to local interactions (local in real space, or in an abstract parameter space) have characteristics dierent from networks formed of random or non-local interactions? Using interacting elds obeying Fokker-Planck equations we show that many network character- istics such as degree distribution, degree-degree correlation and clustering can either be derived analytically or there are analytical bounds on their behaviour. In particular, we derive recursive equations for all powers of the ensemble average of the adjacency matrix. We analyze a few real world networks and show that some networks that seem to form from local interactions indeed have characteristics almost identical to simulations based on our model, in contrast with many other networks.