Stephanos Venakides, Professor
Integrable Systems
Integrable systems mostly consist of families of nonlinear differential equations (ordinary and partial) that can be solved (integrated) in explicit ways through the general principle of the Lax pair, named after its discoverer, Peter Lax. The process of solution has conceptual similarities with the method of the Fourier transform used in the solution of linear differential equations. As in the Fourier transform, there is
a spectral variable at hand. While the solution of linear equations is given by a Fourier integral
in the spectral variable along a certain contour, the nonlinear case is more complicated:
The initial data are used to
specify (a) an oriented contour on the plane of the complex spectral variable and (b) a square "jump" matrix
at each point of the contour. To find the solution to the differential equation, one has to
derive a matrix that (a) is an anlytic function of the spectral variable off the contour, (b)
jumps across the contour, the left limit being equal to the right limit multiplied by the jump matrix,
and (c) has a certain normalization at the infinity point of the spectral variable. Such a problem is known
as a RiemannHilbert problem (RHP). Solving such a problem in the general case is as difficult (indeed, much more so)
as evaluating a general Fourier integral.
The full asymptotic expansion of general Fourier integrals in physically interesting asymptotic limits was made possible by the method of stationary phase/steepest descent, attributed to Lord Kelvin. One encounters such asymptotic limits in calculations of longtime
system behaviors, as well as semiclassical (large frequency or small Planck constant) calculations. The foundation of this approach is that the main contribution from
the integral arises from
the neighborhood of points of the contour of integration where the fast growing exponent
under the integral is stationary. Properly restricted to these neighborhoods, the integral reduces
asymptotically to a Gaussian integral, hence it is readily
computable.
The situation is analogous in the nonlinear case. Through a procedure introduced by Deift and Zhou
in the case of long time limits, factorization of the jump matrix coupled with contour deformations allows the
localization of the contour, the simplification of the jump matrix and the rigorous asymptotic reduction
to a solvable RHP. The procedure is known as steepest descent for RHP, arising from the "pushing"
of parts of the contour to regions where it is exponentially close to the identity and can be thus neglected.
In dispersive equations involving oscillations, the method was readily applicable when the
asymptotic oscillation was weakly nonlinear i.e. consisted of modulated plane wave solutions.
In the presence of fully nonlinear oscillations simply finding the stationary points of a
scalar function was not appropriate. In collaboration with Deift and Zhou, (a) we
found that the reduced or "model" RHP, which
determines the main contribution to the solution has as contour a union of intervals or arcs in
the complex plane, (b) we introduced the "gfunction mechanism", a procedure that led to a system of transcendental equations and inequalities that
the endpoints of the intervals satisfy and from which they are identified uniquely when they exist.
(c) having identified these points, we solved the reduced RHP through a Riemann theta function
and established that the waveform is mostly a modulated quasiperiodic nonlinear wave. This work was done in the context of the celebrated Korteweg de Vries equation (KdV). In joint work, with Deift, Kriecherbauer, McLaughlin (Ken) and Zhou, we implemented this approach to prove an important universality result in the theory of random matrices of the unitary ensemble.
In collaboration with Tovbis and Zhou, we then tackled the problem of the nonlinear
focusing Schroedinger (NLS) equation
that is known to be modulationally unstable (KdV is stable) and thus presented a further difficulty. We have succeeded
in obtaining the global spacetime solution to the initial value problem for special
data that contain only radiation and the solution till the second break in the presence
of a soliton content. In both cases, it is analytic properties of the spectral data (jump matrix)
that save us from the instability. Spectral data NLS calculations are delicate when possible; it required
special work in collaboration with Tovbis to calculate the data in the above cases. Again with Tovbis, we revealed the deeper structure of the modulation equations by bringing them into
a form that involves determinants. We also analyzed the limit of the inverse scattering transform in the asymptotic limit.
What one learns from these theories is that as waveforms evolve, they break into more complicated
waveforms or relax to simpler ones. Multiple theta functions in the formulae describe the
evolution of multiphase modes. The analogue of caustics appears in spacetime along the
boundaries at which the number of participating modes jumps. We have already shown that, with our intial data, there is only one break in the pure radiation case. The local asymptotic analysis of the first break was performed by Bertola and Tovbis.
In collaboration with former student Belov,
we are working to understand the second breaking of the solution of the NLS equation in the presence of solitons, as well as possible subsequent successive breaks that are suggested by numerics. The challenge is that, for a fixed spatial position, we reach a point in time, at which there is an obstacle to our systematic advance of the solution in time. We have made a rigorous asymptotic calculation of the curve in spacetime, along which this difficulty presents itself. We suspect that overcoming this obstacle involves a transformational advance in the asymptotic method itself and we are working in this direction.
Wave Propagation in complex media
In earlier work with Bonilla and Higuera, we studied the breakdown of the stability of the steady state in a Gunn
semiconductor, that leads to the generation of a time periodic pulse train that is commonly used as a microwave
source. With Bonilla Kindelan and Moscoso we analyzed the generation and propagation of traveling fronts in
semiconductor superlattices.
More recently, in collaboration with Haider and Shipman, we studied the scattering of plane waves off a photonic crystal slab, composed of two dielectrics that are distributed periodically along the slab with different refractive indices. We discovered anomalous transmission behavior, as the angle of incidence is varied from normal. With Shipman, we showed that the anomaly is mediated by resonance in the system, in which the incident wave excites a mode along the slab, and we derived an asymptotic formula for the anomalous transmission near the resonant frequency. The formula has very good agreement with the results of simulation. Significantly, the derived profile depends only on a small number of parameters. These few parameters encapsulate all the possible geometric configurations of the photonic crystal.
Most of the materials used in practice, are either linear or weakly nonlinear. However, fully nonlinear phenomena occur near the above resonance, due to the large magnitude of the resonant fields. With Shipman, we constructed and solved a fully nonlinear model displaying such phenomena. The model involves a linear transmission line in which an incident plane wave scatters off a point defect, that is coupled to a nonlinear oscillator. As the coupling increases from zero, a frequency band emerges near the resonant frequency of the defect, in which three (as opposed to one) harmonic solutions are possible. Three solutions also appear at all
frequencies beyond a high frequency threshold.
As the coupling constant is further increased (but is still quite small) the band grows, the threshold frequency diminishes, until the two
3solution regions touch and merge to one.
It was pointed out to us by physicist and applied mathematician, S. Komineas, that our line/oscillator model is of interest in the Bose
Einstein condensation community, for being a simplification of a model for the onset of
vortexantivortex pairs in polariton superfluids in the optical parametric oscillator (OPO)
regime. Our current effort, with Shipman and Komineas, is to develop the mathematical tools for solving this broader model.
Mathematical Biology
Several years ago, I joined the "laser group" of colleagues Dan Kiehart (group leader, Biology) and Glenn Edwards (Physics). The group studies the drosophila dorsal closure (see below) and derives its name from experiments involving laser ablations of the drosophila embryo. The group includes postdocs and graduate students and works through weekly meetings. My interest is the modeling of the closure of the dorsal opening of the drosophila embryo in the process of morphogenesis. The dorsal opening has the shape of a human eye and is only covered by an extraembryonic, epithelial tissue, the amnioserosa; during closure the opposite flanks are "zipped" together at the canthi ("eye" edges). The challenge is to understand the nature of the forces, how they affect the kinetics and their biological and physical origin.
We developed a mathematical model that connects the empirical
kinematic observations with contributing tissue forces, that affect the morphology of the dorsal surface and, in particular, the movements of the purse string and of the canthi. We model the coordinated elastic and contractile motor forces, attributed to the action of actin and myosin that drive DC, by introducing a unit that satisfies a law similar to the law derived by Hill in the early modeling of muscle dynamics. We model the zipping process through a phenomenological law that summarizes the complicated processes of the canthus.
Our model
recapitulates the experimental observations of wild type native,
laser perturbed and mutant native closure made
in earlier work of the group (Hutson et.al.)
The current goal is a transformational extension of the model that (a) will allow deformations that are not restricted to ones that are traverse to the dorsal midline, (b) will introduce the individuality of the amnioserosa cells and (c) will
account for small timescale oscillations observed in the amnioserosa. The scantness of the understanding of the underlying biological mechanism makes this effort quite challenging.
 Contact Info:
Teaching (Fall 2017):
 MATH 353.01, ORD AND PRTL DIFF EQUATIONS
Synopsis
 Physics 235, TuTh 11:45 AM01:00 PM
 MATH 353.04, ORD AND PRTL DIFF EQUATIONS
Synopsis
 Physics 235, TuTh 01:25 PM02:40 PM
Teaching (Spring 2018):
 MATH 353.01, ORD AND PRTL DIFF EQUATIONS
Synopsis
 Physics 119, TuTh 11:45 AM01:00 PM
 MATH 79090.05, MINICOURSE IN ADVANCED TOPICS
Synopsis
 Physics 205, TuTh 01:25 PM02:40 PM
 Office Hours:
 Monday, Wednesday 2pm3pm
 Education:
Ph.D.  New York University  1982 
M.S.  Georgia Institute of Technology  1979 
B.S.  National Technical University of Athens (Greece)  1969 
 Specialties:

Analysis
Applied Math
 Research Interests: Integrable systems, Wave motion in complex media, Mathematical biology
Current projects:
 Analysis of the breaking of the semiclassical focusing nonlinear Schroedinger equation ,
 Resonant phenomena photonic crystals,
 Modeling dorsal closure in drosophila
Integrable Systems
Integrable systems mostly consist of families of nonlinear differential equations (ordinary and partial) that can be solved (integrated) in explicit ways through the general principle of the Lax pair, named after its discoverer, Peter Lax. The process of solution has conceptual similarities with the method of the Fourier transform used in the solution of linear differential equations. As in the Fourier transform, there is
a spectral variable at hand. While the solution of linear equations is given by a Fourier integral
in the spectral variable along a certain contour, the nonlinear case is more complicated:
The initial data are used to
specify (a) an oriented contour on the plane of the complex spectral variable and (b) a square "jump" matrix
at each point of the contour. To find the solution to the differential equation, one has to
derive a matrix that (a) is an anlytic function of the spectral variable off the contour, (b)
jumps across the contour, the left limit being equal to the right limit multiplied by the jump matrix,
and (c) has a certain normalization at the infinity point of the spectral variable. Such a problem is known
as a RiemannHilbert problem (RHP). Solving such a problem in the general case is as difficult (indeed, much more so)
as evaluating a general Fourier integral.
The full asymptotic expansion of general Fourier integrals in physically interesting asymptotic limits was made possible by the method of stationary phase/steepest descent, attributed to Lord Kelvin. One encounters such asymptotic limits in calculations of longtime
system behaviors, as well as semiclassical (large frequency or small Planck constant) calculations. The foundation of this approach is that the main contribution from
the integral arises from
the neighborhood of points of the contour of integration where the fast growing exponent
under the integral is stationary. Properly restricted to these neighborhoods, the integral reduces
asymptotically to a Gaussian integral, hence it is readily
computable.
The situation is analogous in the nonlinear case. Through a procedure introduced by Deift and Zhou
in the case of long time limits, factorization of the jump matrix coupled with contour deformations allows the
localization of the contour, the simplification of the jump matrix and the rigorous asymptotic reduction
to a solvable RHP. The procedure is known as steepest descent for RHP, arising from the "pushing"
of parts of the contour to regions where it is exponentially close to the identity and can be thus neglected.
In dispersive equations involving oscillations, the method was readily applicable when the
asymptotic oscillation was weakly nonlinear i.e. consisted of modulated plane wave solutions.
In the presence of fully nonlinear oscillations simply finding the stationary points of a
scalar function was not appropriate. In collaboration with Deift and Zhou, (a) we
found that the reduced or "model" RHP, which
determines the main contribution to the solution has as contour a union of intervals or arcs in
the complex plane, (b) we introduced the "gfunction mechanism", a procedure that led to a system of transcendental equations and inequalities that
the endpoints of the intervals satisfy and from which they are identified uniquely when they exist.
(c) having identified these points, we solved the reduced RHP through a Riemann theta function
and established that the waveform is mostly a modulated quasiperiodic nonlinear wave. This work was done in the context of the celebrated Korteweg de Vries equation (KdV). In joint work, with Deift, Kriecherbauer, McLaughlin (Ken) and Zhou, we implemented this approach to prove an important universality result in the theory of random matrices of the unitary ensemble.
In collaboration with Tovbis and Zhou, we then tackled the problem of the nonlinear
focusing Schroedinger (NLS) equation
that is known to be modulationally unstable (KdV is stable) and thus presented a further difficulty. We have succeeded
in obtaining the global spacetime solution to the initial value problem for special
data that contain only radiation and the solution till the second break in the presence
of a soliton content. In both cases, it is analytic properties of the spectral data (jump matrix)
that save us from the instability. Spectral data NLS calculations are delicate when possible; it required
special work in collaboration with Tovbis to calculate the data in the above cases. Again with Tovbis, we revealed the deeper structure of the modulation equations by bringing them into
a form that involves determinants. We also analyzed the limit of the inverse scattering transform in the asymptotic limit.
What one learns from these theories is that as waveforms evolve, they break into more complicated
waveforms or relax to simpler ones. Multiple theta functions in the formulae describe the
evolution of multiphase modes. The analogue of caustics appears in spacetime along the
boundaries at which the number of participating modes jumps. We have already shown that, with our intial data, there is only one break in the pure radiation case. The local asymptotic analysis of the first break was performed by Bertola and Tovbis.
In collaboration with former student Belov,
we are working to understand the second breaking of the solution of the NLS equation in the presence of solitons, as well as possible subsequent successive breaks that are suggested by numerics. The challenge is that, for a fixed spatial position, we reach a point in time, at which there is an obstacle to our systematic advance of the solution in time. We have made a rigorous asymptotic calculation of the curve in spacetime, along which this difficulty presents itself. We suspect that overcoming this obstacle involves a transformational advance in the asymptotic method itself and we are working in this direction.
Wave Propagation in complex media
In earlier work with Bonilla and Higuera, we studied the breakdown of the stability of the steady state in a Gunn
semiconductor, that leads to the generation of a time periodic pulse train that is commonly used as a microwave
source. With Bonilla Kindelan and Moscoso we analyzed the generation and propagation of traveling fronts in
semiconductor superlattices.
More recently, in collaboration with Haider and Shipman, we studied the scattering of plane waves off a photonic crystal slab, composed of two dielectrics that are distributed periodically along the slab with different refractive indices. We discovered anomalous transmission behavior, as the angle of incidence is varied from normal. With Shipman, we showed that the anomaly is mediated by resonance in the system, in which the incident wave excites a mode along the slab, and we derived an asymptotic formula for the anomalous transmission near the resonant frequency. The formula has very good agreement with the results of simulation. Significantly, the derived profile depends only on a small number of parameters. These few parameters encapsulate all the possible geometric configurations of the photonic crystal.
Most of the materials used in practice, are either linear or weakly nonlinear. However, fully nonlinear phenomena occur near the above resonance, due to the large magnitude of the resonant fields. With Shipman, we constructed and solved a fully nonlinear model displaying such phenomena. The model involves a linear transmission line in which an incident plane wave scatters off a point defect, that is coupled to a nonlinear oscillator. As the coupling increases from zero, a frequency band emerges near the resonant frequency of the defect, in which three (as opposed to one) harmonic solutions are possible. Three solutions also appear at all
frequencies beyond a high frequency threshold.
As the coupling constant is further increased (but is still quite small) the band grows, the threshold frequency diminishes, until the two
3solution regions touch and merge to one.
It was pointed out to us by physicist and applied mathematician, S. Komineas, that our line/oscillator model is of interest in the Bose
Einstein condensation community, for being a simplification of a model for the onset of
vortexantivortex pairs in polariton superfluids in the optical parametric oscillator (OPO)
regime. Our current effort, with Shipman and Komineas, is to develop the mathematical tools for solving this broader model.
Mathematical Biology
Several years ago, I joined the "laser group" of colleagues Dan Kiehart (group leader, Biology) and Glenn Edwards (Physics). The group studies the drosophila dorsal closure (see below) and derives its name from experiments involving laser ablations of the drosophila embryo. The group includes postdocs and graduate students and works through weekly meetings. My interest is the modeling of the closure of the dorsal opening of the drosophila embryo in the process of morphogenesis. The dorsal opening has the shape of a human eye and is only covered by an extraembryonic, epithelial tissue, the amnioserosa; during closure the opposite flanks are "zipped" together at the canthi ("eye" edges). The challenge is to understand the nature of the forces, how they affect the kinetics and their biological and physical origin.
We developed a mathematical model that connects the empirical
kinematic observations with contributing tissue forces, that affect the morphology of the dorsal surface and, in particular, the movements of the purse string and of the canthi. We model the coordinated elastic and contractile motor forces, attributed to the action of actin and myosin that drive DC, by introducing a unit that satisfies a law similar to the law derived by Hill in the early modeling of muscle dynamics. We model the zipping process through a phenomenological law that summarizes the complicated processes of the canthus.
Our model
recapitulates the experimental observations of wild type native,
laser perturbed and mutant native closure made
in earlier work of the group (Hutson et.al.)
The current goal is a transformational extension of the model that (a) will allow deformations that are not restricted to ones that are traverse to the dorsal midline, (b) will introduce the individuality of the amnioserosa cells and (c) will
account for small timescale oscillations observed in the amnioserosa. The scantness of the understanding of the underlying biological mechanism makes this effort quite challenging.
 Areas of Interest:
 Integrable Systems,
 Wave Propagation and Photonic Crystals,
 Mathematical Biology
 Keywords:
Animals • Animals, Genetically Modified • Cell Adhesion • Computer Simulation • dorsal closure • Drosophila • Drosophila Proteins • Embryo, Nonmammalian • Embryonic Development • Epithelial Cells • Epithelium • Genes, Insect • Image Processing, ComputerAssisted • Integrin alpha Chains • Integrins • Lasers • Mathematics • Mechanotransduction, Cellular • Microscopy, Confocal • Microsurgery • Models, Biological • Morphogenesis • Mutation • Pseudopodia • resonance • semiclassical NLS • Stress, Mechanical • UpRegulation
 Current Ph.D. Students
(Former Students)
 Postdocs Mentored
 Andreas Aristotelous (2012  2013)
 Recent Publications
(More Publications)
 Kiehart, DP; Crawford, JM; Aristotelous, A; Venakides, S; Edwards, GS, Cell Sheet Morphogenesis: Dorsal Closure in Drosophila melanogaster as a Model System.,
Annual Review of Cell and Developmental Biology, vol. 33
(October, 2017),
pp. 169202 [doi] [abs]
 Sergey Belov and Stephanos Venakides, Smooth parametric dependence of asymptotics of the semiclassical focusing NLS,
Analysis & PDE, vol. 8 no. 2
(April, 2015),
pp. 257288 [doi] [abs]
 Komineas, S; Shipman, SP; Venakides, S, Continuous and discontinuous dark solitons in polariton condensates,
Physical Review B  Condensed Matter and Materials Physics, vol. 91 no. 13
(April, 2015), ISSN 10980121 [doi] [abs]
 Belov, S; Venakides, S, Smooth parametric dependence of asymptotics of the semiclassical focusing NLS,
Analysis and PDE, vol. 8 no. 2
(January, 2015),
pp. 257288 [1211.7111v2], [doi] [abs]
 Stavros Komineas, Stephen P. Shipman, Stephanos Venakides, Lossless Polariton Solitons,
arXiv
(Preprint, 2014) [arXiv:1409.4067] [abs]
 Recent Grant Support
 Wavebreaking and Resonant Phenomena, National Science Foundation, DMS1211638, 2012/092018/08.
 Conferences Organized
 SIAM Conference on Analysis of Partial Differential Equations, Minisymposium organizer, Asymptotic behavior of solutions of PDE, December 2007
 "Recent Advances in Partial Differential Equations" in Honor of the 80th Birthdays of P.D. Lax and L. Nirenberg, June, 2006
