Expand all Collapse all  Results 26  50 of 65 
26. CJM 2009 (vol 61 pp. 721)
SubRiemannian Geometry on the Sphere $\mathbb{S}^3$ We discuss the subRiemannian
geometry induced by two noncommutative
vector fields which are left invariant
on the Lie group $\mathbb{S}^3$.
Keywords:noncommutative Lie group, quaternion group, subRiemannian geodesic, horizontal distribution, connectivity theorem, holonomic constraint Categories:53C17, 53C22, 35H20 
27. CJM 2009 (vol 61 pp. 548)
Fundamental Tone, Concentration of Density, and Conformal Degeneration on Surfaces We study the effect of two types of degeneration of a Riemannian
metric on the first eigenvalue of the Laplace operator on
surfaces. In both cases we prove that the first eigenvalue of the
round sphere is an optimal asymptotic upper bound. The first type of
degeneration is concentration of the density to a point within a
conformal class. The second is degeneration of the
conformal class to the boundary of the moduli space on the torus and
on the Klein bottle. In the latter, we follow the outline proposed
by N. Nadirashvili in 1996.
Categories:35P, 58J 
28. CJM 2008 (vol 60 pp. 1168)
Short Time Behavior of Solutions to Linear and Nonlinear Schr{Ã¶dinger Equations We examine the fine structure of the short time behavior
of solutions to various linear and nonlinear Schr{\"o}dinger equations
$u_t=i\Delta u+q(u)$ on $I\times\RR^n$, with initial data $u(0,x)=f(x)$.
Particular attention is paid to cases where $f$ is piecewise smooth,
with jump across an $(n1)$dimensional surface. We give detailed
analyses of Gibbslike phenomena and also focusing effects, including
analogues of the Pinsky phenomenon. We give results for general $n$
in the linear case. We also have detailed analyses for a broad class of
nonlinear equations when $n=1$ and $2$, with emphasis on the analysis of
the first order correction to the solution of the corresponding linear
equation. This work complements estimates on the error in this approximation.
Categories:35Q55, 35Q40 
29. CJM 2008 (vol 60 pp. 822)
Maximum Principles for Subharmonic Functions Via Local SemiDirichlet Forms Maximum principles for subharmonic
functions in the framework of quasiregular local semiDirichlet
forms admitting lower bounds are presented.
As applications, we give
weak and strong maximum principles
for (local) subsolutions of a second order elliptic
differential operator on the domain of Euclidean space under conditions on coefficients,
which partially generalize the results by Stampacchia.
Keywords:positivity preserving form, semiDirichlet form, Dirichlet form, subharmonic functions, superharmonic functions, harmonic functions, weak maximum principle, strong maximum principle, irreducibility, absolute continuity condition Categories:31C25, 35B50, 60J45, 35J, 53C, 58 
30. CJM 2008 (vol 60 pp. 572)
NonSelfadjoint Perturbations of Selfadjoint Operators in Two Dimensions IIIa. One Branching Point This is the third in a series of works devoted to spectral
asymptotics for nonselfadjoint
perturbations of selfadjoint $h$pseudodifferential operators in dimension 2, having a
periodic classical flow. Assuming that the strength $\epsilon$
of the perturbation is in the range $h^2\ll \epsilon \ll h^{1/2}$
(and may sometimes reach even smaller values), we
get an asymptotic description of the eigenvalues in rectangles
$[1/C,1/C]+i\epsilon [F_01/C,F_0+1/C]$, $C\gg 1$, when $\epsilon F_0$ is a saddle point
value of the flow average of the leading perturbation.
Keywords:nonselfadjoint, eigenvalue, periodic flow, branching singularity Categories:31C10, 35P20, 35Q40, 37J35, 37J45, 53D22, 58J40 
31. CJM 2008 (vol 60 pp. 241)
SemiClassical Wavefront Set and Fourier Integral Operators Here we define and prove some properties of the semiclassical
wavefront set. We also define and study semiclassical Fourier
integral operators and prove a generalization of Egorov's theorem to
manifolds of different dimensions.
Keywords:wavefront set, Fourier integral operators, Egorov theorem, semiclassical analysis Categories:35S30, 35A27, 58J40, 81Q20 
32. CJM 2007 (vol 59 pp. 1301)
Strichartz Inequalities for the Wave Equation with the Full Laplacian on the Heisenberg Group We prove dispersive and Strichartz inequalities for the solution of the wave
equation related to the full
Laplacian on the Heisenberg group, by means of Besov spaces defined by a
LittlewoodPaley
decomposition related to the spectral resolution of the full Laplacian.
This requires a careful
analysis due also to the nonhomogeneous nature of the full Laplacian.
This result has to be compared to a previous one by Bahouri, G\'erard
and Xu concerning the solution of the wave equation related to
the Kohn Laplacian.
Keywords:nilpotent and solvable Lie groups, smoothness and regularity of solutions of PDEs Categories:22E25, 35B65 
33. CJM 2007 (vol 59 pp. 943)
A Weighted $L^2$Estimate of the Witten Spinor in Asymptotically Schwarzschild Manifolds We derive a weighted $L^2$estimate of the Witten spinor in
a complete Riemannian spin manifold~$(M^n, g)$ of nonnegative scalar curvature
which is asymptotically Schwarzschild.
The interior geometry of~$M$ enters this estimate only
via the lowest eigenvalue of the square of the Dirac
operator on a conformal compactification of $M$.
Categories:83C60, 35Q75, 35J45, 58J05 
34. CJM 2007 (vol 59 pp. 742)
Geometry and Spectra of Closed Extensions of Elliptic Cone Operators We study the geometry of the set of closed extensions of index $0$ of
an elliptic differential cone operator and its model operator in
connection with the spectra of the extensions, and we give a necessary
and sufficient condition for the existence of rays of minimal growth
for such operators.
Keywords:resolvents, manifolds with conical singularities, spectral theor, boundary value problems, Grassmannians Categories:58J50, 35J70, 14M15 
35. CJM 2007 (vol 59 pp. 393)
Le splitting pour l'opÃ©rateur de KleinGordon: une approche heuristique et numÃ©rique Dans cet article on \'etudie la diff\'erence entre les deux
premi\`eres valeurs propres, le splitting, d'un op\'erateur de
KleinGordon semiclassique unidimensionnel, dans le cas d'un
potentiel sym\'etrique pr\'esentant un double puits. Dans le cas d'une
petite barri\`ere de potentiel, B. Helffer et B. Parisse ont obtenu
des r\'esultats analogues \`a ceux existant pour l'op\'erateur de
Schr\"odinger. Dans le cas d'une grande barri\`ere de potentiel, on
obtient ici des estimations des tranform\'ees de Fourier des fonctions
propres qui conduisent \`a une conjecture du splitting. Des calculs
num\'eriques viennent appuyer cette conjecture.
Categories:35P05, 34L16, 34E05, 47A10, 47A70 
36. CJM 2006 (vol 58 pp. 691)
Hypoelliptic BiInvariant Laplacians on Infinite Dimensional Compact Groups On a compact connected group $G$, consider the infinitesimal
generator $L$ of a central symmetric Gaussian convolution
semigroup $(\mu_t)_{t>0}$. Using appropriate notions of distribution
and smooth function spaces, we prove that $L$ is hypoelliptic if and only if
$(\mu_t)_{t>0} $ is absolutely continuous with respect to Haar measure
and admits a continuous density $x\mapsto \mu_t(x)$, $t>0$, such that
$\lim_{t\ra 0} t\log \mu_t(e)=0$. In particular, this condition holds
if and only if any Borel measure $u$ which is solution of $Lu=0$
in an open set $\Omega$ can be represented by a continuous
function in $\Omega$. Examples are discussed.
Categories:60B15, 43A77, 35H10, 46F25, 60J45, 60J60 
37. CJM 2006 (vol 58 pp. 64)
Multiplicity Results for Nonlinear Neumann Problems In this paper we study nonlinear elliptic problems of Neumann type driven by the
$p$Laplac\ian differential operator. We look for situations guaranteeing the existence
of multiple solutions. First we study problems which are strongly resonant at infinity
at the first (zero) eigenvalue. We prove five multiplicity results, four for problems
with nonsmooth potential and one for problems with a $C^1$potential. In the last part,
for nonsmooth problems in which the potential eventually exhibits a strict
super$p$growth under a symmetry condition, we prove the existence of infinitely
many pairs of nontrivial solutions. Our approach is variational based on the critical
point theory for nonsmooth functionals. Also we present some results concerning the first
two elements of the spectrum of the negative $p$Laplacian with Neumann boundary condition.
Keywords:Nonsmooth critical point theory, locally Lipschitz function,, Clarke subdifferential, Neumann problem, strong resonance,, second deformation theorem, nonsmooth symmetric mountain pass theorem,, $p$Laplacian Categories:35J20, 35J60, 35J85 
38. CJM 2005 (vol 57 pp. 1193)
Some Conditions for Decay of Convolution Powers and Heat Kernels on Groups Let $K$ be a function on a unimodular locally compact group
$G$, and denote by $K_n = K*K* \cdots * K$ the $n$th convolution
power of $K$.
Assuming that $K$ satisfies certain operator estimates in $L^2(G)$,
we give estimates of
the norms $\K_n\_2$ and $\K_n\_\infty$
for large $n$.
In contrast to previous methods for estimating $\K_n\_\infty$,
we do not need to assume that
the function $K$ is a probability density or nonnegative.
Our results also adapt for continuous time semigroups on $G$.
Various applications are given, for example, to estimates of
the behaviour of heat kernels on Lie groups.
Categories:22E30, 35B40, 43A99 
39. CJM 2005 (vol 57 pp. 1291)
Dupin Hypersurfaces in $\mathbb R^5$ We study Dupin
hypersurfaces in $\mathbb R^5$ parametrized by lines of curvature, with
four distinct principal curvatures. We characterize locally a generic
family of such hypersurfaces in terms of the principal curvatures and
four vector valued functions of one variable. We show that these vector
valued functions are invariant by inversions and homotheties.
Categories:53B25, 53C42, 35N10, 37K10 
40. CJM 2005 (vol 57 pp. 771)
The Resolvent of Closed Extensions of Cone Differential Operators We study closed extensions $\underline A$ of
an elliptic differential operator $A$ on a manifold with conical
singularities, acting as an unbounded operator on a weighted $L_p$space.
Under suitable conditions we show that the resolvent
$(\lambda\underline A)^{1}$ exists
in a sector of the complex plane and decays like $1/\lambda$ as
$\lambda\to\infty$. Moreover, we determine the structure of the resolvent
with enough precision to guarantee existence and boundedness of imaginary
powers of $\underline A$.
As an application we treat the LaplaceBeltrami operator for a metric with
straight conical degeneracy and describe domains yielding
maximal regularity for the Cauchy problem $\dot{u}\Delta u=f$, $u(0)=0$.
Keywords:Manifolds with conical singularities, resolvent, maximal regularity Categories:35J70, 47A10, 58J40 
41. CJM 2004 (vol 56 pp. 794)
SemiClassical Behavior of the Scattering Amplitude for Trapping Perturbations at Fixed Energy We study the semiclassical behavior as $h\rightarrow 0$ of the scattering
amplitude $f(\theta,\omega,\lambda,h)$ associated to a Schr\"odinger operator
$P(h)=\frac 1 2 h^2\Delta +V(x)$ with shortrange trapping
perturbations. First we realize a spatial localization in the general case
and we deduce a bound of the scattering amplitude on the real
line. Under an additional assumption on the resonances, we show that
if we modify the potential $V(x)$ in a domain lying behind the
barrier $\{x:V(x)>\lambda\}$, the scattering amplitude
$f(\theta,\omega,\lambda,h)$ changes by a term of order
$\O(h^{\infty})$. Under an escape assumption on the classical
trajectories incoming with fixed direction $\omega$, we obtain
an asymptotic development of $f(\theta,\omega,\lambda,h)$
similar to the one established in thenontrapping case.
Categories:35P25, 35B34, 35B40 
42. CJM 2004 (vol 56 pp. 638)
Multisymplectic Reduction for Proper Actions We consider symmetries of the Dedonder equation arising from
variational problems with partial derivatives. Assuming a proper
action of the symmetry group, we identify a set of reduced equations
on an open dense subset of the domain of definition of the fields
under consideration. By continuity, the Dedonder equation is
satisfied whenever the reduced equations are satisfied.
Keywords:Dedonder equation, multisymplectic structure, reduction,, symmetries, variational problems Categories:58J70, 35A30 
43. CJM 2004 (vol 56 pp. 655)
On the Neumann Problem for the SchrÃ¶dinger Equations with Singular Potentials in Lipschitz Domains We consider the Neumann problem for the Schr\"odinger equations $\Delta u+Vu=0$,
with singular nonnegative potentials $V$ belonging to the reverse H\"older class
$\B_n$, in a connected Lipschitz domain $\Omega\subset\mathbf{R}^n$. Given
boundary data $g$ in $H^p$ or $L^p$ for $1\epsilon

44. CJM 2004 (vol 56 pp. 590)
The Heat Kernel and Green's Function on a Manifold with Heisenberg Group as Boundary We study the Riemannian LaplaceBeltrami operator $L$ on a Riemannian
manifold with Heisenberg group $H_1$ as boundary. We calculate the heat
kernel and Green's function for $L$, and give global and small time
estimates of the heat kernel. A class of hypersurfaces in this
manifold can be regarded as approximations of $H_1$. We also restrict
$L$ to each hypersurface and calculate the corresponding heat kernel
and Green's function. We will see that the heat kernel and Green's
function converge to the heat kernel and Green's function on the
boundary.
Categories:35H20, 58J99, 53C17 
45. CJM 2003 (vol 55 pp. 401)
Gaussian Estimates in Lipschitz Domains We give upper and lower Gaussian estimates for the diffusion kernel of a
divergence and nondivergence form elliptic operator in a Lipschitz domain.
On donne des estimations Gaussiennes pour le noyau d'une diffusion,
r\'eversible ou pas, dans un domaine Lipschitzien.
Categories:39A70, 3502, 65M06 
46. CJM 2002 (vol 54 pp. 1121)
Fully Nonlinear Elliptic Equations on General Domains By means of the Pucci operator, we construct a function $u_0$, which plays
an essential role in our considerations, and give the existence and regularity
theorems for the bounded viscosity solutions of the generalized Dirichlet
problems of second order fully nonlinear elliptic equations on the general
bounded domains, which may be irregular. The approximation method, the accretive
operator technique and the Caffarelli's perturbation theory are used.
Keywords:Pucci operator, viscosity solution, existence, $C^{2,\psi}$ regularity, Dini condition, fully nonlinear equation, general domain, accretive operator, approximation lemma Categories:35D05, 35D10, 35J60, 35J67 
47. CJM 2002 (vol 54 pp. 945)
Approximation on Closed Sets by Analytic or Meromorphic Solutions of Elliptic Equations and Applications 
Approximation on Closed Sets by Analytic or Meromorphic Solutions of Elliptic Equations and Applications Given a homogeneous elliptic partial differential operator $L$ with constant
complex coefficients and a class of functions (jetdistributions) which
are defined on a (relatively) closed subset of a domain $\Omega$ in $\mathbf{R}^n$ and
which belong locally to a Banach space $V$, we consider the problem of
approximating in the norm of $V$ the functions in this class by ``analytic''
and ``meromorphic'' solutions of the equation $Lu=0$. We establish new Roth,
Arakelyan (including tangential) and Carleman type theorems for a large class
of Banach spaces $V$ and operators $L$. Important applications to boundary
value problems of solutions of homogeneous elliptic partial differential
equations are obtained, including the solution of a generalized Dirichlet
problem.
Keywords:approximation on closed sets, elliptic operator, strongly elliptic operator, $L$meromorphic and $L$analytic functions, localization operator, Banach space of distributions, Dirichlet problem Categories:30D40, 30E10, 31B35, 35Jxx, 35J67, 41A30 
48. CJM 2002 (vol 54 pp. 1065)
Large Time Behavior for the Cubic Nonlinear SchrÃ¶dinger Equation We consider the Cauchy problem for the cubic nonlinear Schr\"odinger
equation in one space dimension
\begin{equation}
\begin{cases}
iu_t + \frac12 u_{xx} + \bar{u}^3 = 0,
& \text{$t \in \mathbf{R}$, $x \in \mathbf{R}$,} \\
u(0,x) = u_0(x), & \text{$x \in \mathbf{R}$.}
\end{cases}
\label{A}
\end{equation}
Cubic type nonlinearities in one space dimension heuristically appear
to be critical for large time. We study the global existence and
large time asymptotic behavior of solutions to the Cauchy problem
(\ref{A}). We prove that if the initial data $u_0 \in
\mathbf{H}^{1,0} \cap \mathbf{H}^{0,1}$ are small and such that
$\sup_{\xi\leq 1} \arg \mathcal{F} u_0 (\xi)  \frac{\pi n}{2}
< \frac{\pi}{8}$ for some $n \in \mathbf{Z}$, and $\inf_{\xi\leq
1} \mathcal{F} u_0 (\xi) >0$, then the solution has an additional
logarithmic timedecay in the short range region $x \leq
\sqrt{t}$. In the far region $x > \sqrt{t}$ the asymptotics have
a quasilinear character.
Category:35Q55 
49. CJM 2002 (vol 54 pp. 998)
Resonances for Slowly Varying Perturbations of a Periodic SchrÃ¶dinger Operator We study the resonances of the operator $P(h) = \Delta_x + V(x) +
\varphi(hx)$. Here $V$ is a periodic potential, $\varphi$ a
decreasing perturbation and $h$ a small positive constant. We prove
the existence of shape resonances near the edges of the spectral bands
of $P_0 = \Delta_x + V(x)$, and we give its asymptotic expansions in
powers of $h^{\frac12}$.
Categories:35P99, 47A60, 47A40 
50. CJM 2002 (vol 54 pp. 493)
Perverse Sheaves on Grassmannians We compute the category of perverse sheaves on Hermitian symmetric
spaces in types~A and D, constructible with respect to the Schubert
stratification. The calculation is microlocal, and uses the action of
the Borel group to study the geometry of the conormal variety
$\Lambda$.
Keywords:perverse sheaves, microlocal geometry Categories:32S60, 32C38, 35A27 