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Harmonic mappings in the plane are univalent complex-valued harmonic functions of a complex variable. Conformal mappings are a special case where the real and imaginary parts are conjugate harmonic functions, satisfying the Cauchy-Riemann equations. Harmonic mappings were studied classically by differential geometers because they provide isothermal (or conformal) parameters for minimal surfaces. More recently they have been actively investigated by complex analysts as generalizations of univalent analytic functions, or conformal mappings. Many classical results of geometric function theory extend to harmonic mappings, but basic questions remain unresolved. This book is the first comprehensive account of the theory of planar harmonic mappings, treating both the generalizations of univalent analytic functions and the connections with minimal surfaces.  Essentially self-contained, the book contains background material in complex analysis and a full development of the classical theory of minimal surfaces, including the Weierstrass-Enneper representation. It is designed to introduce non-specialists to a beautiful area of complex analysis and geometry.

/pdf/harmonic-mappings-in-the-plane-39i0l3cser.pdf

Harmonic Mappings in the Plane

Can one hear the shape of a drum

Volume II J. S. R. Chisholm and Rosa M. Morris Amsterdam: North-Holland. 1964 Pp. xviii + 717. Price 72s. This book is a valuable addition to an ever-growing collection of mathematical texts written expressly for the physicist. The authors suggest that the book might also be useful to undergraduates in chemistry and engineering and research workers in other fields, but the content is clearly chosen to meet the needs of modern undergraduate physics.

http://iopscience.iop.org/article/10.1088/0031-9112/16/8/015/pdf

Mathematical Methods in Physics

Finite element methods for incompressible viscous flow

We study the linear stability of smooth steady states of the evolution equation

$$h_t = - (f(h)_{xxx} )_x - (g(h)h_x )_x - ah$$

under both periodic and Neumann boundary conditions. If a≠ 0 we assume f≡ 1. In particular we consider positive periodic steady states of thin film equations, where a=0 and f, g might have degeneracies such as f(0)=0 as well as singularities like g(0)=+∞.

https://www.math.toronto.edu/mpugh/Prints/LinearStability/linear_stability.pdf

Linear Stability of Steady States for Thin Film and Cahn-Hilliard Type Equations

A subgroup D of GL (n, ℝ) is said to be admissible if the semidirect product of D and ℝn, considered as a subgroup of the affine group on ℝn, admits wavelets ψ ∈ L2(ℝn) satisfying a generalization of the Calderon reproducing, formula. This article provides a nearly complete characterization of the admissible subgroups D. More precisely, if D is admissible, then the stability subgroup Dx for the transpose action of D on ℝn must be compact for a. e. x. ∈ ℝn; moreover, if Δ is the modular function of D, there must exist an a ∈ D such that |det a| ≠ Δ(a). Conversely, if the last condition holds and for a. e. x ∈ ℝn there exists an e > 0 for which the e-stabilizer Dxe is compact, then D is admissible. Numerous examples are given of both admissible and non-admissible groups.

A characterization of the higher dimensional groups associated with continuous wavelets

(1996). The Argument Principle for Harmonic Functions. The American Mathematical Monthly: Vol. 103, No. 5, pp. 411-415.

The argument principle for harmonic functions

We consider nonnegative steady-state solutions of the evolution equationformula hereOur class of coefficients f, g allows degeneracies at h = 0, such as f(0) = 0, as well as divergences like g(0) = ±∞. We first construct steady states and study their regularity. For f, g > 0 we construct positive periodic steady states, and non-negative steady states with either zero or nonzero contact angles. For f > 0 and g < 0, we prove there are no non-constant positive periodic steady states or steady states with zero contact angle, but we do construct non-negative steady states with nonzero contact angle. In considering the volume, length (or period) and contact angle of the steady states, we find a rescaling identity that enables us to answer questions such as whether a steady state is uniquely determined by its volume and contact angle. Our tools include an improved monotonicity result for the period function of the nonlinear oscillator. We also relate the steady states and their scaling properties to a recent blow-up conjecture of Bertozzi and Pugh.

Properties of steady states for thin film equations

We consider nonnegative steady-state solutions of the evolution equation formula here Our class of coefficients f, g allows degeneracies at h = 0, such as f(0) = 0, as well as divergences like g(0) = ±∞. We first construct steady states and study their regularity. For f, g > 0 we construct positive periodic steady states, and non-negative steady states with either zero or nonzero contact angles. For f > 0 and g < 0, we prove there are no non-constant positive periodic steady states or steady states with zero contact angle, but we do construct non-negative steady states with nonzero contact angle. In considering the volume, length (or period) and contact angle of the steady states, we find a rescaling identity that enables us to answer questions such as whether a steady state is uniquely determined by its volume and contact angle. Our tools include an improved monotonicity result for the period function of the nonlinear oscillator. We also relate the steady states and their scaling properties to a recent blow-up conjecture of Bertozzi and Pugh.

Richard S. Laugesen

Papers

Linear Stability of Steady States for Thin Film and Cahn-Hilliard Type Equations

A characterization of the higher dimensional groups associated with continuous wavelets

The argument principle for harmonic functions

Properties of steady states for thin film equations

Properties of Steady States for Thin Film Equations