# Numerical Schemes from the Perspective of Consensus

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1 Numerical Schemes from the Perspective of Consensus Exploring Connections between Agreement Problems and PDEs Department of Electrical Engineering and Computer Sciences University of California, Berkeley April 27, 2010

2 Characterizing Numerical Schemes through Consensus Given a general PDE, how can we analyze it from the p.o.v. of Linear Systems Theory? Particularly interested in parabolic and elliptic PDEs that involve the Laplace operator Au xx + Bu xy + Cu yy +... = 0, where B 2 4AC 0 For example, heat equation: u t = au xx Why the Laplace operator? Rich theory of graphs utilizing discrete Lapace matrix Concise framework through which to understand many problems in discrete distributed control Question: which classes of PDE control problems can be studied as consensus problems and how can this framework help us?

3 Outline of this talk General overview of the objectives of the project Mathematical Preliminaries Consensus on Networks Control of Spatially Varying Interconnected Systems Illustrative example studying a PDE system as a consensus problem Broader discussion of the framework and potential future directions

4 Outline Preliminaries 1 Preliminaries

5 Numerical Methods and Control Numerical Analysis well-understood, lends itself to solution of real ODE/PDE systems Simplest example: First-order Euler explicit forward dierence (from Taylor series) u = f (t, u) u t+h = u t + h(t, u t )

6 How Robust is the Scheme? Stability: Backward or implicit dierence schemes stable for any h Sti Systems[1] Accuracy vs. eciency: take into account more terms, tighter granularity in step size Choice may depend on operating constraints, e.g., online or oine

7 Numerical Schemes as Linear Systems For systems, linear discretization results in an LTI system: u = Su + Q, u, Q R n, S R n n Standard linear systems tests apply for stability. See if S is Hurwitz, etc.

8 Some Working Denitions for Graphs Denition A vertex v is a point in n dimensional space. Denition An edge is an ordered pair (v 1, v 2 ) where v i are vertices connected by a segment. Denition A graph G = G(V, E) is a collection of vertices V connected by the set of edges E.

9 Graph Models[3] Preliminaries Cycle Randomly Generated Graph

10 The Graph Laplacian Denition The degree of a vertex is the number of edges of which it is a member. Denition A graph Laplacian is a square matrix dened as follows[2]: d i : i = j [L] ij = 1 : e ij = (v i, v j ) E 0 : otherwise (1)

11 Laplacian Spectral Analysis Laplacian is symmetric positive semidenite At least one eigenvalue at zero, with corresponding eigenvector [1... 1] T Spectrum of Laplacian describes network dynamics Fiedler eigenvalue[1]: smallest positive eigenvalue a measure of connectedness

12 The Agreement Problem Given a collection of agents that can communicate Characterize conditions required for agreement on a parameter Describe convergence properties, e.g., asymptotic, exponential,...

13 Outline Preliminaries 1 Preliminaries

14 Consensus Problem on Networks Olfati-Saber, et al., Given a collection of nodes, model their interaction by ẋ = Lx, continuous case x(k + 1) = (I ɛl)x(k), discrete case Theorem For a continuous-time system, global exponential consensus is reached with speed at least as fast as λ 2 (L). For a discrete-time system, global exponential consensus is reached with speed at least as fast as 1 ɛλ 2 (L), provided that ɛ < 1 d max, where d max is the maximum degree of any vertex of L.

15 Consensus Problem on Networks, II Fascinating emergent behavior: group of nodes behaves like a continuum reaching a steady-state value Applications[3] Formation control Coupled oscillators Flocking Small world networks Distributed sensor fusion

16 Control of Spatially Varying Systems[3] D'Andrea and Dullerud, Spatially varying system Denes necessary and sucient LMI conditions for stabilization Example application: discretization of the heat equation in two dimensions Our goal: understand discretization from the perspective of consensus

17 Control of Spatially Varying Systems[3] D'Andrea and Dullerud, Spatially varying system Denes necessary and sucient LMI conditions for stabilization Example application: discretization of the heat equation in two dimensions Our goal: understand discretization from the perspective of consensus

18 Control of Spatially Varying Systems[4], II Finite linear connection Finite planar connection 1 1 D'Andrea, Langbort, and Chandra, 2003.

19 Outline Preliminaries 1 Preliminaries

20 Heat Equation on the Unit Circle Heat equation on the circle[4] parametrized by x [0, 1) with θ = 2πx: u t = au xx Separation of variables readily yields: u(x, t) = (T 0 H t )(x) = n= a n e 4π2 n 2 t e 2πjnx H t (x) is the heat kernel for the unit circle and a n are the Fourier coecients of T 0 Eigenvalues given by 4π 2 n 2

21 Spatial Representation[3] Circle 2 (1D Periodic BC) 2 D'Andrea and Dullerud, 2003.

22 Three point stencil 1D Laplacian: second-order centered dierence scheme. (u t ) i = a x 2 (u i+1 2u i + u i 1 ) Resulting space discretization: rst order linear ODE Same dynamics as the consensus problem!

23 Laplacian Graph Structure and Interpretation To impose periodic boundary conditions[2], note that u 0 = u N. The system representations are L = a x u = Lu u(k + 1) = (I ɛl)u(k).

24 Spectral Analysis Preliminaries Using discrete Fourier transform, with the eigenfunctions f (j) (x) = e Ik j i x, we get λ(φ m ) = 2a x 2 (cos φ m 1), with φ m = Ik m x = 2mπ, m = 0,... N 1. So the eigenvalues N range in value from 4a x 2 to zero. Let a = 1 and x = 1. Then: N ( 2 λ(l) = 1 2mπ ) x 2 cos N 4m 2 π 2.

25 Spectral Analysis, II Eigenvalues of discretization converge to eigenvalues of continuous operator. λ 2 (L) 4π 2 Space-discretized ODE will reach globally exponential consensus value with a speed at least as great as 4π 2. Standard ODE time integration methods chosen based on location of eigenvalues Key idea: The original PDE system, discretized, is exactly the consensus problem for a cycle topology.

26 Spectral Analysis, II Eigenvalues of discretization converge to eigenvalues of continuous operator. λ 2 (L) 4π 2 Space-discretized ODE will reach globally exponential consensus value with a speed at least as great as 4π 2. Standard ODE time integration methods chosen based on location of eigenvalues Key idea: The original PDE system, discretized, is exactly the consensus problem for a cycle topology.

27 Outline Preliminaries 1 Preliminaries

28 Dierent Schemes Preliminaries Use dierent discretizations 5, 7, 9 point stencils Algebraic connectivity improves Computational cost increases For a large grid, stencil size will be less important Modied 3 point stencil: look two steps behind and ahead Doesn't have much eect on connectivity

29 Extensions to higher dimensions Same idea applicable to higher spatial order Laplacian terms u t = a u Apply higher order spatial discretizations 2D case: typical choice 5 or 9 point scheme (u t ) i, j = a x 2 (u i+1, j + u i 1, j + u i, j+1 + u i, j 1 4u i, j )

30 Visual Representation[3] Torus 3 (2D Periodic BC) 3 D'Andrea and Dullerud, 2003.

31 Stencil Grids Preliminaries One spatial dimension Two spatial dimensions

32 Classes of Systems Amenable to Consensus Graph Analysis Periodic Boundary Conditions Neumann Boundary Conditions Can control through the boundaries Control terms can also be applied at each actuator: turns out that if there are at least two independent inputs (outputs), LTI system is controllable (observable). General LTI form: u = Lu + Gr

33 Outline Preliminaries 1 Preliminaries

34 Horizons Preliminaries Other Boundary Conditions: cannot be directly represented as graph Laplacian Potential future work in this direction: how can these systems be understood as consensus problems? Can use linear state feedback to get to Laplacian form, but may not be realistic Designing better interconnection structures through graph theory Boyd et al.: Convex optimization to design fastest mixing Markov chians, minimize resistance in a network, etc.

35 Summary Preliminaries Many second-order PDEs can be analyzed in a consensus framework Consensus a useful paradigm for distributed control Example: Heat Equation Laplacian graph theory and consensus: Facilitate better numerical schemes Aid in understanding complex continuum dynamics

36 Additional Reading R. L. Burden and J. D. Faires. Numerical Analysis, F. Chung, Spectral Graph Theory, 1997 R. D'Andrea and G. Dullerud. "Distributed Control Design for Spatially Interconnected Systems," R. D'Andrea, C. Langbort, and R. Chandra. "A State Space Approach to Control of Interconnected Systems," 2003.

37 Additional Reading, II M. Fiedler. "Algebraic Connectivity of Graphs," C. Hirsch. Computational Fluid Dynamics, R. Olfati-Saber, et al. "Consensus and Cooperation in Multiagent Systems," E. M. Stein and R. Shakarchi. Introduction to Fourier Analysis, 2003.

38 Questions Preliminaries Thank You Any Questions?

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