Colloquium: Pamela Harris (Williams)
Nov 1 @ 3:30 pm – 4:30 pm
Colloquium- Yevhen Zelenyuk @ Keller 401
Nov 8 @ 3:30 pm – 4:30 pm

Speaker: Yevhen Zelenyuk (Wits. University)

Title: Discontinuity of multiplication and left translations in $G^{LUC}$

Abstract: Every locally compact group $G$ has a largest semigroup
compactification, denoted $G^{LUC}$. For every $p\in G$ and $q\in G^{LUC}$,
the multiplication in $G^{LUC}$ is continuous at $(p,q)$ (Ellis 1957), and
for every $p\in G^*=G^{LUC}\setminus G$, the left translation by $p$ in
$G^*$ is discontinuous (Protasov and Pym 2001). We shall discuss the
question whether there are $p,q\in G^*$ such that the multiplication in
$G^{LUC}$ is continuous at $(p,q)$ or the left translation by $p$ in $G^*$
is continuous at $q$.

Semnar Chekroun @ Keller Hall
Nov 13 @ 3:30 pm – 4:30 pm

Title: On stochastic parameterizing manifolds: Pullback characterization and Non-Markovian reduced equations

Abstract: In this talk, a general approach to provide approximate parameterizations of the “small” scales by the “large” ones, will be presented for stochastic partial differential equations (SPDEs) driven by linear multiplicative noise. This is accomplished via the concept of parameterizing manifolds (PMs) that are stochastic manifolds which improve in mean square error the partial knowledge of the full SPDE solution u when compared to the projection of u onto the resolved modes, for a given realization of the noise.
Backward-forward systems are designed to give access to such PMs in practice. The key idea consists of representing the modes with high wave numbers (as parameterized by the sought PM) as a pullback limit depending on the time-history of the modes with low wave numbers.
The resulting manifolds obtained by such a procedure are not subject to a spectral gap condition such as encountered in the classical theory. Instead, certain PMs can be determined under weaker non-resonance conditions.
Non-Markovian stochastic reduced systems are then derived based on such a PM approach. Such reduced systems take the form of SDEs involving random coefficients that convey memory effects via the history of the Wiener process, and arise from the nonlinear interactions between the low modes, embedded in the “noise bath.” These random coefficients follow typically non-Gaussian statistics and exhibit an exponential decay of correlations whose rate depends explicitly on gaps arising in the non-resonances conditions.
It is shown on a stochastic Burgers-type equation, that such PM-based reduced systems can achieve very good performance in reproducing statistical features of the SPDE dynamics projected onto the resolved modes, such as the autocorrelations and probability functions of the corresponding modes amplitude, even when the latter is large and the amount of noise is significant.

Colloquium- Jim McElwaine (U. Durham) @ Keller 401
Nov 15 @ 3:30 pm – 4:30 pm

Title: Granular Flows and Martian Gullies

Speaker: Prof. Jim McElwaine, Department of Earth Sciences, University of Durham


Dense granular flows occur frequently in both nature and industry,
yet, despite their prevalence, they remain poorly understood. Most
theories are empirically based and are unreliable when applied outside
their narrow range of validity. For terrestrial phenomena this is only
an inconvenience as more detailed experiments always be performed. For
extra-terrestrial phenomena however this is largely impossible and
likely to remain so. For this reason it is essential to develop
physically based theories that can be applied throughout the solar
systems where gravity, air pressure and temperature may have very
different values. I report on chute and drum experiments of granular
flows and explain how these observations are applicable in
interpreting observations. As a case study I focus on the flow of
carbon dioxide blocks down Martian dunes.

Colloquium-Markus Pflaum (U. Colorado) @ Keller 401
Nov 22 @ 3:30 pm – 4:30 pm

Speaker: Markus Pflaum (U. Colorado)

Title: Whitney Functions, the real homotopy type of a semi-analytic set, and a
Hochschild-Kostant-Rosenberg type theorem

In the talk we consider semi-analytic subsets of a real analytic
manifold and their homology and real homotopy type.
It is well-known that de Rham’s Theorem does not hold true in general
for singular spaces such as semi-analytic sets.
We show that to remedy this one can replace the de Rham complex by the
Whitney-de Rham complex to compute the
singular homology of such sets. Beyond that, the Whitney-deRham complex
even determines the real homotopy type of
a semi-analytic set which extends a result by Sullivan for the de Rham
complex on smooth manifolds.
Finally, we comment on a Hochschild-Kostant-Rosenberg type theorem for
Whitney functions.
The talk is based on joint work with B. Chriestenson and J.-P.

Master’s Defense: Ms. Erica Brown @ Keller 301
Dec 2 @ 3:00 pm – 5:00 pm
Colloquium: Pamela Harris (Williams)
Dec 6 @ 3:30 pm – 4:30 pm
Colloquium: Pamela Harris (Williams)
Jan 3 @ 3:30 pm – 4:30 pm