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Kevin Zumbrun

Publications and source records attributed to Kevin Zumbrun.

At least 19 recordsLinked to original sources
arXiv2025

Non-linear stability of shock profiles in dissipative hyperbolic-hyperbolic systems

We give the first proof of nonlinear stability for smooth shock profiles of second-order dissipative hyperbolic-hyperbolic systems under the assumption of spectral stability, showing stability of smooth small-amplitude profiles in dimensions greater than or equal to two. This class of systems notably includes the two types of causal viscous relativistic gas models introduced respectively by Freist\"uhler-Temple and Bemfica-Disconzi-Noronha, and (the equivalent second-order form of) a class of first-order numerical relaxation systems generalizing the well-known Jin-Xin relaxation schemes. A significant technical innovation is a new para-differential type of nonlinear damping estimate similar to that used by the first author to study stability of constant states, allowing the treatment of systems far from the symmetric structure required for the standard ``Kawashima-type'' energy estimates that are typically used for that purpose.

math.AP↗
arXiv2025

Linear damping estimates for periodic roll wave solutions of the inviscid Saint-Venant equations and related systems of hyperbolic balance laws

Substantially extending previous results of the authors for smooth solutions in the viscous case, we develop linear damping estimates for periodic roll-wave solutions of the inviscid Saint-Venant equations and related systems of hyperbolic balance laws. Such damping estimates, consisting of $H^s$ energy estimates yielding exponential slaving of high-derivative to low-derivative norms, have served as crucial ingredients in nonlinear stability analyses of traveling waves in hyperbolic or partially parabolic systems, both in obtaining high-frequency resolvent estimates and in closing a nonlinear iteration for which available linearized stability estimates apparently lose regularity. Here, we establish for systems of size $n\leq 6$ a Lyapunov-type theorem stating that such energy estimates are available whenever strict high-frequency spectral stability holds; for dimensions 7 and higher, there may be in general a gap between high-frequency spectral stability and existence of the type of energy estimate that we develop here. A key ingredient is a dimension-dependent linear algebraic lemma reminiscent of Lyapunov's Lemma for ODE that is to our knowledge new.

math.AP↗
arXiv2025

Nonconvex optimization and convergence of stochastic gradient descent, and solution of asynchronous game

We review convergence and behavior of stochastic gradient descent for convex and nonconvex optimization, establishing various conditions for convergence to zero of the variance of the gradient of the objective function, and presenting a number of simple examples demonstrating the approximate evolution of the probability density under iteration, including applications to both classical two-player and asynchronous multiplayer games

math.OC↗
arXiv2024

Sychronous vs. asynchronous coalitions in multiplayer games, with applications to guts poker

We study the issue introduced by Buck-Lee-Platnick-Wheeler-Zumbrun of synchronous vs. asynchronous coalitions in multiplayer games, that is, the difference between coalitions with full and partial communication, with a specific interest in the context of continuous Guts poker where this problem was originally formulated. We observe for general symmetric multiplayer games, with players 2-n in coalition against player 1, that there are three values, corresponding to symmetric Nash equilibrium, optimal asynchronous, and optimal synchronous strategies, in that order, for which inequalities may for different examples be strict or nonstrict (i.e., equality) in any combination. Different from Nash equilibria and synchronous optima, which may be phrased as convex optimization problems, or classical 2-player games, determination of asynchronous optima is a nonconvex optimization problem. We discuss methods of numerical approximation of this optimum, and examine performance on 3-player rock-paper-scissors and discretized Guts poker. Finally, we present sufficient conditions guaranteeing different possibilities for behavior, based on concave/convexity properties of the payoff function. These answer in the affirmative the open problem posed by Buck-Lee-Platnick-Wheeler-Zumbrun whether the optimal asynchronous coalition value for 3-player guts is equal to the Nash equilibrium value zero. At the same time, we present a number of new results regarding synchronous coalition play for continuous $3$-player guts.

cs.GT↗
arXiv2024

Pseudodifferential damping estimates and stability of relaxation shocks

A bottleneck in the theory of large-amplitude and multi-d viscous and relaxation shock stability is the development of nonlinear damping estimates controlling higher by lower derivatives. These have traditionally proceeded from time-evolution bounds based on Friedrichs symmetric and Kawashima or Goodman type energy estimates. Here, we propose an alternative program based on frequency-dependent pseudodifferential time-space damping estimates in the spirit of Kreiss. These are seen to be equivalent in the linear case to high-frequency spectral stability, and, just as for the constant-coefficient analysis of Kreiss, sharp in a pointwise, fixed-frequency, sense. This point of view leads to a number of simplifications and extensions using already-existing analysis. We point to the new issue of turning points, analogous to glancing points in the constant-coefficient case as an important direction for further development.

math.AP↗
arXiv2024

Linear stability analysis for a system of singular amplitude equations arising in biomorphology

We study linear stability of exponential periodic solutions of a system of singular amplitude equations associated with convective Turing bifurcation in the presence of conservation laws, as arises in modern biomorphology models, binary fluids, and elsewhere. Consisting of a complex Ginzburg-Landau equation coupled with a singular convection-diffusion equation in "mean modes" associated with conservation laws, these were shown previously by the authors to admit a constant-coefficient linearized stability analysis as in the classical Ginzburg-Landau case -- albeit now singular in wave amplitude epsilon -- yielding useful necessary conditions for stability, both of the exponential functions as solutions of the amplitude equations, and of the associated periodic pattern solving the underlying PDE. Here, we show by a delicate two-parameter matrix perturbation analysis that (strict) satisfaction of these necessary conditions is also sufficient for diffusive stability in the sense of Schneider, yielding a corresponding result, and nonlinear stability, for the underlying PDE. Moreover, we show that they may be interpreted as stability along a non-normally hyperbolic slow manifold approximated by Darcy-type reduction, together with attraction along transverse mean modes, connecting with finite-time approximation theorems of Hacker-Schneider-Zimmerman.

math.AP↗
arXiv2023

Game-theoretic analysis of Guts Poker

We carry out a game-theoretic analysis of the recursive game "Guts," a variant of poker featuring repeated play with possibly growing stakes. An interesting aspect of such games is the need to account for funds lost to all players if expected stakes do not go to zero with the number of rounds of play. We provide a sharp, easily applied criterion eliminating this scenario, under which one may compute a value for general games of this type. Using this criterion, we determine an optimal "pure" strategy for the 2-player game consisting of a simple "go/no-go" criterion. For the $n$-player game, $n\geq 3$, we determine an optimal go/no-go strategy against "bloc play" in which players 2-n pursue identical strategies, giving nonnegative return for player 1. Against general collaborative strategies of players 2-n, we show that player 1 cannot force a nonnegative return. It follows that there exists a nonstrict symmetric Nash equilbrium, but this equilibrium is not strong

math.OC↗
arXiv2023

Spectral decomposition and decay to grossly determined solutions for a simplified BGK model

Extending work of Carty, we show that $H^1$ solutions of a simplified 1D BGK model decay exponentially in $L^2$ to a subclass of the class of grossly determined solutions as defined by Truesdell and Muncaster. In the process, we determine the spectrum and generalized eigenfunctions of the associated non-selfadjoint linearized operator and derive the associated generalized Fourier transform and Parseval's identity. Notably, our analysis makes use of rigged space techniques originating from quantum mechanics, as adapted by Ljance and others to the nonselfadjoint case.

math.AP↗
arXiv2023

Multidimensional stability and transverse bifurcation of hydraulic shocks and roll waves in open channel flow

We study by a combination of analytical and numerical methods multidimensional stability and transverse bifurcation of planar hydraulic shock and roll wave solutions of the inviscid Saint Venant equations for inclined shallow-water flow, both in the whole space and in a channel of finite width, obtaining complete stability diagrams across the full parameter range of existence. Technical advances include development of efficient multi-d Evans solvers, low- and high-frequency asymptotics, explicit/semi-explicit computation of stability boundaries, and rigorous treatment of channel flow with wall-type physical boundary. Notable behavioral phenomena are a novel essential transverse bifurcation of hydraulic shocks to invading planar periodic roll-wave or doubly-transverse periodic herringbone patterns, with associated metastable behavior driven by mixed roll- and herringbone-type waves initiating from localized perturbation of an unstable constant state; and Floquet-type transverse ``flapping'' bifurcation of roll wave patterns.

math.AP↗
arXiv2023

Existence and stability of nonmonotone hydraulic shocks for the Saint Venant equations of inclined thin-film flow

Extending work of Yang-Zumbrun for the hydrodynamically stable case of Froude number F < 2, we categorize completely the existence and convective stability of hydraulic shock profiles of the Saint Venant equations of inclined thin-film flow. Moreover, we confirm by numerical experiment that asymptotic dynamics for general Riemann data is given in the hydrodynamic instability regime by either stable hydraulic shock waves, or a pattern consisting of an invading roll wave front separated by a finite terminating Lax shock from a constant state at plus infinity. Notably, profiles, and existence and stability diagrams are all rigorously obtained by mathematical analysis and explicit calculation.

math.AP↗
arXiv2023

Convective Turing bifurcation with conservation laws

Generalizing results of \cite{MC,S} and \cite{HSZ} for certain model reaction-diffusion and reaction-convection-diffusion equations, we derive and rigorously justify weakly nonlinear amplitude equations governing general Turing bifurcation in the presence of conservation laws. In the nonconvective, reaction-diffusion case, this is seen similarly as in \cite{MC,S} to be a real Ginsburg-Landau equation coupled with a diffusion equation in a large-scale mean-mode vector comprising variables associated with conservation laws. In the general, convective case, by contrast, the amplitude equations as noted in \cite{HSZ} consist of a complex Ginsburg-Landau equation coupled with a singular convection-diffusion equation featuring rapidly-propagating modes with speed $\sim 1/\eps$ where $\eps$ measures amplitude of the wave as a disturbance from a background steady state. Different from the partially coupled case considered in \cite{HSZ} in the context of Bénard-Marangoni convection/inclined flow, the Ginzburg Landau and mean-mode equations are here fully coupled, leading to substantial new difficulties in the analysis. Applications are to biological morphogenesis, in particular vasculogenesis, as described by the Murray-Oster and other mechanochemical/hydrodynamical models

math.AP↗
arXiv2023

Existence and stability of steady noncharacteristic solutions on a finite interval of full compressible Navier-Stokes equations

We treat the 1D shock tube problem, establishing existence of steady solutions of full (nonisentropic) polytropic gas dynamics with arbitrary noncharacteristic data. We present also numerical experiments indicating uniqueness and time-asymptotic stability of such solutions. At the same time, we give an example of an (artificial) equation of state possessing a convex entropy for which there holds nonuniqueness of solutions. This is associated with instability and Hopf bifurcation to time-periodic solutions. In a second part, we study general systems of viscous conservation laws and we state results on existence, stability and spectral stability of steady states in restricted cases.

math.AP↗
arXiv2022

Continuous guts poker and numerical optimization of generalized recursive games

We study a type of generalized recursive game introduced by Castronova, Chen, and Zumbrun featuring increasing stakes, with an emphasis on continuous guts poker and $1$ v. $n$ coalitions. Our main results are to develop practical numerical algorithms with rigorous underlying theory for the approximation of optimal mutiplayer strategies, and to use these to obtain a number of interesting observations about guts. Outcomes are a striking 2-strategy optimum for $n$-player coalitions, with asymptotic advantage approximately $16\%$; convergence of Fictitious Play to symmetric Nash equilibrium; and a malevolent interactive $n$-player "bot" for demonstration.

math.OC↗
arXiv2022

Convective-wave solutions of the Richard-Gavrilyuk model for inclined shallow water flow

We study for the Richard-Gavrilyuk model of inclined shallow water flow, an extension of the classical Saint Venant equations incorporating vorticity, the new feature of convective-wave solutions analogous to contact discontinuitis in inviscid conservation laws. These are traveling waves for which fluid velocity is constant and equal to the speed of propagation of the wave, but fluid height and/or enstrophy (thus vorticity) varies. Together with hydraulic shocks, they play an important role in the structure of Riemann solutions.

math.AP↗
arXiv2022

Forward-modulated damping estimates and nonlocalized stability of periodic Lugiato-Lefever wave

In an interesting recent analysis, Haragus-Johnson-Perkins-de Rijk have shown modulational stability under localized perturbations of steady periodic solutions of the Lugiato-Lefever equation (LLE), in the process pointing out a difficulty in obtaining standard "nonlinear damping estimates" on modulated perturbation variables to control regularity of solutions. Here, we point out that in place of standard "inverse-modulated" damping estimates, one can alternatively carry out a damping estimate on the "forward-modulated" perturbation, noting that norms of forward- and inverse-modulated variables are equivalent modulo absorbable errors, thus recovering the classical argument structure of Johnson-Noble-Rodrigues-Zumbrun for parabolic systems. This observation seems of general use in situations of delicate regularity. Applied in the context of (LLE) it gives the stronger result of stability and asymptotic behavior with respect to nonlocalized perturbations.

math.AP↗
arXiv2022

Instantaneous smoothing and exponential decay of solutions for a degenerate evolution equation with application to Boltzmann's equation

We establish an instantaneous smoothing property for decaying solutions on the half-line $(0,+\infty)$ of certain degenerate Hilbert space-valued evolution equations arising in kinetic theory, including in particular the steady Boltzmann equation. Our results answer the two main open problems posed by Pogan and Zumbrun in their treatment of $H^1$ stable manifolds of such equations, showing that $L^2_{loc}$ solutions that remain sufficiently small in $L^\infty$ (i) decay exponentially, and (ii) are $C^\infty$ for $t>0$, hence lie eventually in the $H^1$ stable manifold constructed by Pogan and Zumbrun

math.AP↗
arXiv2022

Large-amplitude modulation of periodic traveling waves

We introduce a new approach to the study of modulation of high-frequency periodic wave patterns, based on pseudodifferential analysis, multi-scale expansion, and Kreiss symmetrizer estimates like those in hyperbolic and hyperbolic-parabolic boundary-value theory. Key ingredients are local Floquet transformation as a preconditioner removing large derivatives in the normal direction of background rapidly oscillating fronts and the use of the periodic Evans function of Gardner to connect spectral information on component periodic waves to block structure of the resulting approximately constant-coefficient resolvent ODEs. Our main result is bounded-time existence and valitidy to all orders of large-amplitude smooth modulations of planar periodic solutions of multi-D reaction diffusion systems in the high-frequency/small wavelength limit.

math.AP↗