Using the computer to select the right variables

1. Lake Carnegie, Princeton, NJ Using the computer to select the right variables Rationale: Straight Line Distance Curved Transition Distance Actual transition difficulty represented by curved path

2. Z X Y Using the computer to choose the right variables Rationale: 3D Dataset with 2D manifold Selected Datapoint Euclidean Distance Euclidean distance in input space may be weak indicator of INTRINSIC similarity of datapoints Geodesic distance is good for this dataset

3. Diffusion Maps Dataset in x, y, z Dataset Diffusion Map eigencomputation N datapoints N datapoints R. Coifman, S. Lafon, A. Lee, M. Maggioni, B. Nadler, F. Warner, and S. Zucker, Geometric diffusions as a tool for harmonic analysis and structure definition of data: Diffusion maps. PNAS 102 (2005). B. Nadler, S. Lafon, R. Coifman, and I. G. Kevrekidis, Diffusion maps, spectral clustering and reaction coordinates of dynamical systems. Appl. Comput. Harmon. Anal. 21 (2006).

4. Diffusion Map (2, 3) Report absolute distance of all uninformed individuals to informed individual to DMAP routine Report (signed) distance of all uninformed individuals to informed individual to DMAP routine ABSOLUTE Coordinates SIGNED Coordinates Reaction Coordinate 3 STICK STICK SLIP SLIP 2 2

5. So, again, the same simple theme • If there is some reason to believe that there exist slow, effective dynamics in some smart collective variables • Then this can be used to accelerate some features of the computation • Tools for data-based detection of coarse variables… • BUT • You can start COMPUTATIONS wherever you want • You CANNOT (not easily!) start experiments wherever you want • TALK to experiments with spatiotemporal resolution

6. Effective simplicity • Construct predictive models (deterministic, Markovian) • Get information from them: CALCULUS, Taylor series • Derivatives in time to jump in time • Derivatives in parameter space for sensitivity /optimization • Derivatives in phase space for contraction mappings • Derivatives in physical space for PDE discretizations In complex systems --- no derivatives at the level we need them sometimes no variables ---- no calculus If we know what the right variables are, we can PERFORM differential operations on the right variables – A Calculus for Complex Systems

7. Coming full circle 1. No equations ? Isn’t that a little medieval ? Equations =“Understanding” AGAIN matrix free iterative linear algebra A x = b PRECONDITIONING, B A x = B b B approximate inverse of A Use “the best equation you have” to precondition equation-free computations. 2. With enough initialization authority: equation free laboratory experiments

8. Computer-Aided Analysisof Nonlinear Problems in Transport PhenomenaRobert A. Brown, L. E. Scriven and William J. Sillimanin HOLMES, P.J., New Approaches to Nonlinear Problems in Dynamics, 1980 ABSTRACTThe nonlinear partial differential equations of mass, momentum, energy, Species and charge transport…. can be solved in terms of functions of limited differentiability, no more than the physics warrants, rather than the analytic functions of classical analysis… ….. basis sets consisting of low-order polynomials. …. systematically generating and analyzing solutions by fast computers employing modern matrix techniques. ….. nonlinear algebraic equations by the Newton-Raphson method. … The Newton-Raphson technique is greatly preferred because the Jacobian of the solution is a treasure trove, not only for continuation, but also for analysing stability of solutions, for detecting bifurcations of solution families, and for computing asymptotic estimates of the effects, on any solution, of small changes in parameters, boundary conditions, and boundary shape…… In what we do, not only the analysis, but the equations themselves are obtained on the computer, from short experiments with an alternative, microscopic description.