6,447 research outputs found
A Primer on Reproducing Kernel Hilbert Spaces
Reproducing kernel Hilbert spaces are elucidated without assuming prior
familiarity with Hilbert spaces. Compared with extant pedagogic material,
greater care is placed on motivating the definition of reproducing kernel
Hilbert spaces and explaining when and why these spaces are efficacious. The
novel viewpoint is that reproducing kernel Hilbert space theory studies
extrinsic geometry, associating with each geometric configuration a canonical
overdetermined coordinate system. This coordinate system varies continuously
with changing geometric configurations, making it well-suited for studying
problems whose solutions also vary continuously with changing geometry. This
primer can also serve as an introduction to infinite-dimensional linear algebra
because reproducing kernel Hilbert spaces have more properties in common with
Euclidean spaces than do more general Hilbert spaces.Comment: Revised version submitted to Foundations and Trends in Signal
Processin
Stochastic firing rate models
We review a recent approach to the mean-field limits in neural networks that
takes into account the stochastic nature of input current and the uncertainty
in synaptic coupling. This approach was proved to be a rigorous limit of the
network equations in a general setting, and we express here the results in a
more customary and simpler framework. We propose a heuristic argument to derive
these equations providing a more intuitive understanding of their origin. These
equations are characterized by a strong coupling between the different moments
of the solutions. We analyse the equations, present an algorithm to simulate
the solutions of these mean-field equations, and investigate numerically the
equations. In particular, we build a bridge between these equations and
Sompolinsky and collaborators approach (1988, 1990), and show how the coupling
between the mean and the covariance function deviates from customary
approaches
Mean Field description of and propagation of chaos in recurrent multipopulation networks of Hodgkin-Huxley and Fitzhugh-Nagumo neurons
We derive the mean-field equations arising as the limit of a network of
interacting spiking neurons, as the number of neurons goes to infinity. The
neurons belong to a fixed number of populations and are represented either by
the Hodgkin-Huxley model or by one of its simplified version, the
Fitzhugh-Nagumo model. The synapses between neurons are either electrical or
chemical. The network is assumed to be fully connected. The maximum
conductances vary randomly. Under the condition that all neurons initial
conditions are drawn independently from the same law that depends only on the
population they belong to, we prove that a propagation of chaos phenomenon
takes places, namely that in the mean-field limit, any finite number of neurons
become independent and, within each population, have the same probability
distribution. This probability distribution is solution of a set of implicit
equations, either nonlinear stochastic differential equations resembling the
McKean-Vlasov equations, or non-local partial differential equations resembling
the McKean-Vlasov-Fokker- Planck equations. We prove the well-posedness of
these equations, i.e. the existence and uniqueness of a solution. We also show
the results of some preliminary numerical experiments that indicate that the
mean-field equations are a good representation of the mean activity of a finite
size network, even for modest sizes. These experiment also indicate that the
McKean-Vlasov-Fokker- Planck equations may be a good way to understand the
mean-field dynamics through, e.g., a bifurcation analysis.Comment: 55 pages, 9 figure
Equilibrium composition between liquid and clathrate reservoirs on Titan
Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by
the Cassini spacecraft to cover the polar regions of Titan. A significant
fraction of these lakes or seas could possibly be interconnected with
subsurface liquid reservoirs of alkanes. In this paper, we investigate the
interplay that would happen between a reservoir of liquid hydrocarbons located
in Titan's subsurface and a hypothetical clathrate reservoir that progressively
forms if the liquid mixture diffuses throughout a preexisting porous icy layer.
To do so, we use a statistical-thermodynamic model in order to compute the
composition of the clathrate reservoir that forms as a result of the
progressive entrapping of the liquid mixture. This study shows that clathrate
formation strongly fractionates the molecules between the liquid and the solid
phases. Depending on whether the structure I or structure II clathrate forms,
the present model predicts that the liquid reservoirs would be mainly composed
of either propane or ethane, respectively. The other molecules present in the
liquid are trapped in clathrates. Any river or lake emanating from subsurface
liquid reservoirs that significantly interacted with clathrate reservoirs
should present such composition. On the other hand, lakes and rivers sourced by
precipitation should contain higher fractions of methane and nitrogen, as well
as minor traces of argon and carbon monoxide.Comment: Accepted for publication in Icaru
Repentir: Digital exploration beneath the surface of an oil painting
Repentir is a mobile application that employs marker-less tracking and augmented reality to enable gallery visitors to explore the under drawing and successive stages of pigment beneath an oil painting's surface. Repentir recognises the position and orientation of a specific painting within a photograph and precisely overlays images that were captured during that painting's creation. The viewer may then browse through the work's multiple states and closely examine its painted surface in one of two ways: sliding or rubbing. Our current prototype recognises realist painter Nathan Walsh's most recent work, "Transamerica". Repentir enables the viewer to explore intermediary stages in the painting's development and see what is usually lost within the materially additive painting process. The prototype offers an innovative approach to digital reproduction and provides users with unique insights into the painter's working method
Carbon-rich planet formation in a solar composition disk
The C--to--O ratio is a crucial determinant of the chemical properties of
planets. The recent observation of WASP 12b, a giant planet with a C/O value
larger than that estimated for its host star, poses a conundrum for
understanding the origin of this elemental ratio in any given planetary system.
In this paper, we propose a mechanism for enhancing the value of C/O in the
disk through the transport and distribution of volatiles. We construct a model
that computes the abundances of major C and O bearing volatiles under the
influence of gas drag, sublimation, vapor diffusion, condensation and
coagulation in a multi--iceline 1+1D protoplanetary disk. We find a gradual
depletion in water and carbon monoxide vapors inside the water's iceline with
carbon monoxide depleting slower than water. This effect increases the gaseous
C/O and decreases the C/H ratio in this region to values similar to those found
in WASP 12b's day side atmosphere. Giant planets whose envelopes were accreted
inside the water's iceline should then display C/O values larger than those of
their parent stars, making them members of the class of so-called ``carbon-rich
planets''.Comment: 8 pages, 4 figures, accepted for publication Ap
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