Dynamics of meromorphic maps with small topological degree III: geometric currents and ergodic theory

We continue our study of the dynamics of mappings with small topological degree on (projective) complex surfaces. Previously, under mild hypotheses, we have constructed an ergodic ``equilibrium'' measure for each such mapping. Here we study the dynamical properties of this measure in detail: we give optimal bounds for its Lyapunov exponents, prove that it has maximal entropy, and show that it has product structure in the natural extension. Under a natural further assumption, we show that saddle points are equidistributed towards this measure. This generalize results that were known in the invertible case and is, to our knowledge, one among not very many instances in which a natural invariant measure for a non-invertible dynamical system is well-understood.Comment: v3. Exposition improved. Final version, to appear in Ann. Scient. de l'EN

Dynamics of meromorphic maps with small topological degree I: from cohomology to currents

We consider the dynamics of a meromorphic map on a compact kahler surface whose topological degree is smaller than its first dynamical degree. The latter quantity is the exponential rate at which its iterates expand the cohomology class of a kahler form. Our goal in this article and its sequels is to carry out a conjectural program for constructing and analyzing a natural measure of maximal entropy for each such map. Here we take the first step, converting information about the linear action of the map on cohomology to invariant currents with special geometric structure. We also give some examples and identify some additional properties of maps on irrational surfaces and of maps whose invariant cohomology classes have vanishing self-intersection.Comment: Final version, to appear in Indiana University Mathematics Journal. Among other changes, discussion of polynomial maps is improve

French birds lag behind climate warming

Biodiversity responses to climate warming have been documented through the study of changes in distributions, abundances or phenologies of individual species or in more integrated measures such as species community richness and composition. However, whether these observed population and community changes are occurring fast enough to cope with new climatic conditions remain uncertain and hardly quantifiable. Here, using spatial and temporal trends from the French breeding bird survey, we show that although bird assemblages are strongly responding to climate warming, this response is slower than expected for catching up with the current temperature increase. During the last two decades, French birds have only achieved 54% of the response required to follow temperature increase, and have accumulated, in 18 years, a 97 km delay in their northward shift. We thus developed a framework to measure both the observed and predicted response of species assemblage to climate change, an approach which is flexible enough to be applicable to any taxa with large-scale survey data, using either abundance or distribution data. For example, it can be further used to test if different delays are found across groups or if, for a given group, the delay depends on the land-use contexts

Rank-1 Constrained Multichannel Wiener Filter for Speech Recognition in Noisy Environments

Multichannel linear filters, such as the Multichannel Wiener Filter (MWF) and the Generalized Eigenvalue (GEV) beamformer are popular signal processing techniques which can improve speech recognition performance. In this paper, we present an experimental study on these linear filters in a specific speech recognition task, namely the CHiME-4 challenge, which features real recordings in multiple noisy environments. Specifically, the rank-1 MWF is employed for noise reduction and a new constant residual noise power constraint is derived which enhances the recognition performance. To fulfill the underlying rank-1 assumption, the speech covariance matrix is reconstructed based on eigenvectors or generalized eigenvectors. Then the rank-1 constrained MWF is evaluated with alternative multichannel linear filters under the same framework, which involves a Bidirectional Long Short-Term Memory (BLSTM) network for mask estimation. The proposed filter outperforms alternative ones, leading to a 40% relative Word Error Rate (WER) reduction compared with the baseline Weighted Delay and Sum (WDAS) beamformer on the real test set, and a 15% relative WER reduction compared with the GEV-BAN method. The results also suggest that the speech recognition accuracy correlates more with the Mel-frequency cepstral coefficients (MFCC) feature variance than with the noise reduction or the speech distortion level.Comment: for Computer Speech and Languag

Foreground-Background Ambient Sound Scene Separation

Ambient sound scenes typically comprise multiple short events occurring on top of a somewhat stationary background. We consider the task of separating these events from the background, which we call foreground-background ambient sound scene separation. We propose a deep learning-based separation framework with a suitable feature normaliza-tion scheme and an optional auxiliary network capturing the background statistics, and we investigate its ability to handle the great variety of sound classes encountered in ambient sound scenes, which have often not been seen in training. To do so, we create single-channel foreground-background mixtures using isolated sounds from the DESED and Audioset datasets, and we conduct extensive experiments with mixtures of seen or unseen sound classes at various signal-to-noise ratios. Our experimental findings demonstrate the generalization ability of the proposed approach

Spatial coherence of monsoon onset over Western and Central Sahel (1950-2000)

International audienceThe spatial coherence of boreal monsoon onset over Western and Central Sahel (Senegal, Mali, Burkina-Faso) is studied through the analysis of daily rainfall data for 103 stations from 1950 to 2000. Onset date is defined using a local agronomic definition, i.e. the first wet day (> 1 mm) of 1 or 2 consecutive days receiving at least 20 mm without a 7-day dry spell receiving less than 5 mm in the following 20 days. Changing either the length and/or the amplitude of the initial wet spell, or the length of the following dry spell modify the long-term mean local-scale onset date but has only a weak impact either on its interannual variability or its spatial coherence. Onset date exhibits a seasonal progression from southern Burkina-Faso (mid May) to northwestern Senegal and Saharian edges (early August). Interannual variability of the local-scale onset date does not seem to be strongly spatially coherent. The amount of common or covariant signal across the stations is far weaker than the inter-station noise at the interannual time scale. In particular, a systematic spatially-consistent, advance or delay of the onset is hardly observed across the whole Western and Central Sahel. In consequence, the seasonal predictability of local-scale onset over the Western and Central Sahel associated for example with large-scale sea surface temperatures, is, at best, weak