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Random Walks and Diffusion >> Content Detail



Syllabus



Syllabus

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Prerequisite


18.305 (Advanced Analytic Methods in Science and Engineering) or permission of the instructor. A basic understanding of probability, partial differential equations, transforms, complex variables, asymptotic analysis, and computer programming would be helpful, but an ambitious student could take the class to learn some of these topics. Interdisciplinary registration is encouraged.



Problem Sets


There are five problem sets for this course. Solutions should be clearly explained. You are encouraged to work in groups and consult various references (but not solutions to problem sets from a previous term), although you must prepare each solution independently, in your own words.



Midterm Exam


There will be one take-home midterm exam. It will be handed out in class and will be due at the next session.



Final Project


There is no final exam, only a written final-project report, due at the last lecture. The topic must be selected and approved six weeks earlier.



Grading



activitiespercentages
Problem Sets40%
Midterm Exam30%
Final Project30%



Topics


  1. Normal Diffusion (12+ Lectures)
    • Central Limit Theorem, Asymptotic Approximations, Drift and Dispersion, Fokker-Planck Equation, First Passage, Return, Exploration.
  2. Anomalous Diffusion (10+ Lectures)
    • Non-identical Steps, Persistence and Self Avoidance, Levy Flights, Continuous Time Random Walk, Fractional Diffusion Equations, Random Environments.
  3. Nonlinear Diffusion (4 Lectures, As Time Permits)
    • Interacting Walkers, Steric Effects, Electrolytes, Porous Media, DLA.


Recommended Texts


Amazon logo Hughes, B. Random Walks and Random Environments. Vol. 1. Oxford, UK: Clarendon Press, 1996. ISBN: 0198537883.

Amazon logo Redner, S. A Guide to First Passage Processes. Cambridge, UK: Cambridge University Press, 2001. ISBN: 0521652480.

Amazon logo Risken, H. The Fokker-Planck Equation. 2nd ed. New York, NY: Springer-Verlag, 1989. ISBN: 0387504982.



Further Readings


Amazon logo Bouchaud, J. P., and M. Potters. Theory of Financial Risks. Cambridge, UK: Cambridge University Press, 2000. ISBN: 0521782325.

Amazon logo Crank, J. Mathematics of Diffusion. 2nd ed. Oxford, UK: Clarendon Press, 1975. ISBN: 0198533446.

Amazon logo Rudnick, J., and G. Gaspari. Elements of the Random Walk. Cambridge, UK: Cambridge University Press, 2004. ISBN: 0521828910.

Amazon logo Spitzer, F. Principles of the Random Walk. 2nd ed. New York, NY: Springer-Verlag, 2001. ISBN: 0387951547.



Calendar



LEC #TOPICSKEY DATES
1

Overview

History (Pearson, Rayleigh, Einstein, Bachelier)

Normal vs. Anomalous Diffusion

Mechanisms for Anomalous Diffusion

I. Normal Diffusion
I.A. Linear Diffusion
2

Moments, Cumulants, and Scaling


Markov Chain for the Position (in d Dimensions), Exact Solution by Fourier Transform, Moment and Cumulant Tensors, Additivity of Cumulants, "Square-root Scaling" of Normal Diffusion

3

The Central Limit Theorem and the Diffusion Equation


Multi-dimensional CLT for Sums of IID Random Vectors

Continuum Derivation Involving the Diffusion Equation

4

Asymptotic Shape of the Distribution


Berry-Esseen Theorem

Asymptotic Analysis Leading to Edgeworth Expansions, Governing Convergence to the CLT (in one Dimension), and more Generally Gram-Charlier Expansions for Random Walks

Width of the Central Region when Third and Fourth Moments Exist

5

Globally Valid Asymptotics


Method of Steepest Descent (Saddle-Point Method) for Asymptotic Approximation of Integrals

Application to Random Walks

Example: Asymptotics of the Bernoulli Random Walk

Problem set 1 due
6

Power-law "Fat Tails"


Power-law Tails, Diverging Moments and Singular Characteristic Functions

Additivity of Tail Amplitudes

7

Asymptotics with Fat Tails


Corrections to the CLT for Power-law Tails (but Finite Variance)

Parabolic Cylinder Functions and Dawson's Integral

A Numerical Example Showing Global Accuracy and Fast Convergence of the Asymptotic Approximation

8

From Random Walks to Diffusion


Examples of Random Walks Modeled by Diffusion Equations

Flagellar Bacteria

Run and Tumble Motion, Chemotaxis

Financial Time Series

Additive Versus Multiplicative Processes

Problem set 2 due
9

Discrete Versus Continuous Stochastic Processes


Corrections to the Diffusion Equation Approximating Discrete Random Walks with IID Steps

Fat Tails and Riesz Fractional Derivatives

Stochastic Differentials, Wiener Process

10

Weakly Non-identical Steps


Chapman-Kolmogorov Equation, Kramers-Moyall Expansion, Fokker-Planck Equation

Probability Flux

Modified Kramers-Moyall Cumulant Expansion for Identical Steps

I.B. Nonlinear Diffusion
11

Nonlinear Drift


Interacting Random Walkers, Concentration-dependent Drift

Nonlinear Waves in Traffic Flow, Characteristics, Shocks, Burgers' Equation

Surface Growth, Kardar-Parisi-Zhang Equation

12

Nonlinear Diffusion


Cole-Hopf Transformation, General Solution of Burgers Equation

Concentration-dependent Diffusion, Chemical Potential. Rechargeable Batteries, Steric Effects

I.C. First Passage and Exploration
13

Return Probability on a Lattice


Probability Generating Functions on the Integers, First Passage and Return on a Lattice, Polya's Theorem

14

The Arcsine Distribution


Reflection Principle and Path Counting for Lattice Random Walks, Derivation of the Discrete Arcsine Distribution for the Fraction of Time Spent on One Side of the Origin, Continuum Limit

Problem set 3 due
15

First Passage in the Continuum Limit


General Formulation in One Dimension

Smirnov Density

Minimum First Passage Time of a Set of N Random Walkers

16

First Passage in Arbitrary Geometries


General Formulation in Higher Dimensions, Moments of First Passage Time, Eventual Hitting Probability, Electrostatic Analogy for Diffusion, First Passage to a Sphere

17

Conformal Invariance


Conformal Transformations (Analytic Functions of the Plane, Stereographic Projection from the Plane to a Sphere,...), Conformally Invariant Transport Processes (Simple Diffusion, Advection-diffusion in a Potential Flow,...), Conformal Invariance of the Hitting Probability

18

Hitting Probabilities in Two Dimensions


Potential Theory using Complex Analysis, Mobius Transformations, First Passage to a Line

Problem set 4 due
19

Applications of Conformal Mapping


First Passage to a Circle, Wedge/Corner, Parabola. Continuous Laplacian Growth, Polubarinova-Galin Equation, Saffman-Taylor Fingers, Finite-time Singularities

Midterm exam out
20

Diffusion-limited Aggregation


Harmonic Measure, Hastings-Levitov Algorithm, Comparison of Discrete and Continuous Dynamics

Overview of Mechanisms for Anomalous Diffusion. Non-identical Steps

Midterm exam due
II. Anomalous Diffusion
II.A. Breakdown of the CLT
21

Polymer Models: Persistence and Self-avoidance


Random Walk to Model Entropic Effects in Polymers, Restoring Force for Stretching; Persistent Random Walk to Model Bond-bending Energetic Effects, Green-Kubo Relation, Persistence Length, Telegrapher's Equation; Self-avoiding Walk to Model Steric Effects, Fisher-Flory Estimate of the Scaling Exponent

22

Levy Flights


Superdiffusion and Limiting Levy Distributions for Steps with Infinite Variance, Examples, Size of the Largest Step, Frechet Distribution

II.B. Continuous-Time Random Walks
23

Continuous-time Random Walks


Laplace Transform

Renewal Theory

Montroll-Weiss Formulation of CTRW

DNA Gel Electrophoresis

24

Fractional Diffusion Equations


CLT for CTRW

Infinite Man Waiting Time, Mittag-Leffler Decay of Fourier Modes, Time-delayed Flux, Fractional Diffusion Equation

25

Non-separable Continuous-time Random Walks


"Phase Diagram" for Anomalous Diffusion: Large Steps Versus Long Waiting Times

Application to Flagellar Bacteria

Hughes' General Formulation of CTRW with Motion between "turning points"

Problem set 5 due
26

Leapers and Creepers


Hughes' Leaper and Creeper Models

Leaper Example: Polymer Surface Adsorption Sites and Cross-sections of a Random Walk

Creeper Examples: Levy Walks, Bacterial Motion, Turbulent Dispersion


 








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