1,119 research outputs found
Michel Henon, a playfull and simplifying mind
Several chapters in this book present various aspects of Michel Henon's
scientific acheivements that spread over a large range of subjects, and yet
managed to make deep contributions to most of them. The authors of these
chapters make a much better job at demonstrating the big advancements that
Michel Henon allowed in these fields than I could ever do. Here I rather
present some facets of his personnality that most appealed to me. Michel Henon
was a reserved person, almost shy, so it was not obvious for a young student to
grasp the profoundness of his insight and what a marvelous advisor he could be.
The two most prominent aspects of his mind, in my view, were his ability to
simplify any scientific question to its core complexity, and to find the fun
and amusing part in his everyday work, even in the tiniest details of his
scientific investigations.Comment: Invited talk at the "hommage a Michel Henon" conference held at the
"Institut Henri Poincare" Paris, GRAVASCO trimestre, autumn 2013. Proceeding
by Hermann Press, editors: Jean-Michel Alimi, Roya Mohayaee, Jerome Perez.
Videos of all talks are available at:
https://www.youtube.com/playlist?list=PL9kd4mpdvWcBLrN8u04IZW6-akNgLG7-
Polynomial approximation of Poincare maps for Hamiltonian system
Different methods are proposed and tested for transforming a non-linear differential system, and more particularly a Hamiltonian one, into a map without integrating the whole orbit as in the well-known Poincare return map technique. We construct piecewise polynomial maps by coarse-graining the phase-space surface of section into parallelograms and using either only values of the Poincare maps at the vertices or also the gradient information at the nearest neighbors to define a polynomial approximation within each cell. The numerical experiments are in good agreement with both the real symplectic and Poincare maps
SQL Query Completion for Data Exploration
Within the big data tsunami, relational databases and SQL are still there and
remain mandatory in most of cases for accessing data. On the one hand, SQL is
easy-to-use by non specialists and allows to identify pertinent initial data at
the very beginning of the data exploration process. On the other hand, it is
not always so easy to formulate SQL queries: nowadays, it is more and more
frequent to have several databases available for one application domain, some
of them with hundreds of tables and/or attributes. Identifying the pertinent
conditions to select the desired data, or even identifying relevant attributes
is far from trivial. To make it easier to write SQL queries, we propose the
notion of SQL query completion: given a query, it suggests additional
conditions to be added to its WHERE clause. This completion is semantic, as it
relies on the data from the database, unlike current completion tools that are
mostly syntactic. Since the process can be repeated over and over again --
until the data analyst reaches her data of interest --, SQL query completion
facilitates the exploration of databases. SQL query completion has been
implemented in a SQL editor on top of a database management system. For the
evaluation, two questions need to be studied: first, does the completion speed
up the writing of SQL queries? Second , is the completion easily adopted by
users? A thorough experiment has been conducted on a group of 70 computer
science students divided in two groups (one with the completion and the other
one without) to answer those questions. The results are positive and very
promising
Extended Dualization: Application to Maximal Pattern Mining
International audienceThe dualization in arbitrary posets is a well-studied problem in combinatorial enumeration and is a crucial step in many applications in logics, databases, artificial intelligence and pattern mining.The objective of this paper is to study reductions of the dualization problem on arbitrary posets to the dualization problem on boolean lattices, for which output quasi-polynomial time algorithms exist. Quasi-polynomial time algorithms are algorithms which run in no(logn) where n is the size of the input and output. We introduce convex embedding and poset reflection as key notions to characterize such reductions. As a consequence, we identify posets, which are not boolean lattices, for which the dualization problem remains in quasi-polynomial time and propose a classification of posets with respect to dualization.From these results, we study how they can be applied to maximal pattern mining problems. We deduce a new classification of pattern mining problems and we point out how known problems involving sequences and conjunctive queries patterns, fit into this classification. Finally, we explain how to adapt the seminal Dualize & Advance algorithm to deal with such patterns.As far as we know, this is the first contribution to explicit non-trivial reductions for studying the hardness of maximal pattern mining problems and to extend the Dualize & Advance algorithm for complex patterns
Expectations for the Deep Impact collision from cometary nuclei modelling
Using the cometary nucleus model developed by Espinasse et al. (1991), we
calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of
360 years. Starting from an initially amorphous cometary nucleus which
incorporates an icy mixture of H2O and CO, we show that, at the time of Deep
Impact collision, the crater is expected to form at depths where ice is in its
crystalline form. Hence, the subsurface exposed to space should not be
primordial. We also attempt an order-of-magnitude estimate of the heating and
material ablation effects on the crater activity caused by the 370 Kg
projectile released by the DI spacecraft. We thus show that heating effects
play no role in the evolution of crater activity. We calculate that the CO
production rate from the impacted region should be about 300-400 times higher
from the crater resulting from the impact with a 35 m ablation than over the
unperturbed nucleus in the immediate post-impact period. We also show that the
H2O production rate is decreased by several orders of magnitude at the crater
base just after ablation
Dualization on Partially Ordered Sets: Preliminary Results
International audienceThe dualization problem on arbitrary posets is a crucial step in many applications in logics, databases, artificial intelligence and pattern mining. The objective of this paper is to study reductions of the dualization problem on arbitrary posets to the dualization problem on boolean lattices, for which output quasi-polynomial time algorithms exist. We introduce convex embedding and poset reflection as key notions to characterize such reductions. As a consequence, we identify posets, which are not boolean lattices, for which the dualization problem remains quasi-polynomial and propose a classification of posets with respect to dualization. As far as we know, this is the first contribution to explicit non-trivial reductions for studying the hardness of dualization problems on arbitrary posets
KBO binaries: are they really primordial ?
Given the large orbital separation and high satellite-to-primary mass ratio
of all known Kuiper Belt Object (KBO) binaries, it is important to reassess
their stability as bound pairs with respect to several disruptive mechanisms.
Besides the classical shattering and dispersing of the secondary due to a
high-velocity impact, we considered the possibility that the secondary is
kicked off its orbit by a direct collision of a small impactor, or that it is
gravitationally perturbed due to the close approach of a somewhat larger TNO.
Depending on the values for the size/mass/separation of the binaries that we
used, 2 or 3 of the 8 pairs can be dispersed in a timescale shorter than the
age of the solar system in the current rarefied environment. A contemporary
formation scenario could explain why we still observe these binaries, but no
convincing mechanism has been proposed to date. The primordial formation
scenarios, which seem to be the only viable ones, must be revised to increase
the formation efficiency in order to account for this high dispersal rate.
Objects like the large-separatioKBO binary n2001 QW must have been
initially an order of magnitude more numerous.
If the KBO binaries are indeed primordial, then we show that the mass
depletion of the Kuiper belt cannot result from collisional grinding, but must
rather be due to dynamical ejection.Comment: Submitted to Icarus, Mai 9th, 200
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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