297 research outputs found
The Onsager--Machlup functional for data assimilation
When taking the model error into account in data assimilation, one needs to
evaluate the prior distribution represented by the Onsager--Machlup functional.
Through numerical experiments, this study clarifies how the prior distribution
should be incorporated into cost functions for discrete-time estimation
problems. Consistent with previous theoretical studies, the divergence of the
drift term is essential in weak-constraint 4D-Var (w4D-Var), but it is not nec
essary in Markov chain Monte Carlo with the Euler scheme. Although the former
property may cause difficulties when implementing w4D-Var in large systems,
this paper proposes a new technique for estimating the divergence term and its
derivative.Comment: Reprint from Nonlin. Processes Geophys. (ver.5). 12 pages, 5 figure
Principal Geodesic Analysis applied to climate time series
Principal Geodesic Analysis (PGA) is applied to a climate time series. First,
we transform each multidimensional sequence into the path signature. Since the
signature lives in a curved space, usual principal component analysis (PCA) is
not applicable. Instead, we treat the signature space as a geodesic manifold.
By replacing the notion of straight lines with that of geodesics, the domain of
PCA can be extended to the curved space. Then, the first principal component is
derived as the geodesic that minimizes the unexplained variance in the data. As
an application, we computed the leading modes for one-year NINO SST time
series, which are divided into segments that represent annual variations of
monthly averages. It is interesting that some months in a year reveal
characteristic undulations that could indicate the early signs of upcoming El
Nin\~{o} events.Comment: draft, 23pages, 21figure
Machine learning technique using the signature method for automated quality control of the Argo profiles
A profile from the Argo ocean observation array is a sequence of
three-dimensional vectors composed of pressure, salinity, and temperature,
appearing as a continuous curve in three-dimensional space. The shape of this
curve is faithfully represented by a path signature, which is a collection of
all the iterated integrals. Moreover, the product of two terms of the signature
of a path can be expressed as the sum of higher-order terms. Thanks to this
algebraic property, a nonlinear function of profile shape can always be
represented by a weighted linear combination of the iterated integrals, which
enables machine learning of a complicated function of the profile shape. In
this study, we performed supervised learning for existing Argo data with
quality control flags by using the signature method, and demonstrated the
estimation performance by cross-validation. Unlike rule-based approaches, which
require several complicated and possibly subjective rules, this method is
simple and objective in nature because it relies only on past knowledge
regarding the shape of profiles. This technique should be critical to realizing
automatic quality control for Argo profile data.Comment: 21 pages. 13 figures, 1 table, in revisio
The barycenter in free nilpotent Lie groups and its application to iterated-integrals signatures
We establish well-definedness of the barycenter for every integrable measure
on the free nilpotent Lie group of step (over ). Our main
motivation stems from measures induced by iterated-integrals signatures.Comment: 49 pages, 1 figur
B cell-derived GABA elicits IL-10⁺ macrophages to limit anti-tumour immunity
GABAを標的とする抗腫瘍免疫機構 --代謝産物を介した免疫細胞間制御の一端を解明--. 京都大学プレスリリース. 2021-11-10.Small, soluble metabolites not only are essential intermediates in intracellular biochemical processes, but can also influence neighbouring cells when released into the extracellular milieu1-3. Here we identify the metabolite and neurotransmitter GABA as a candidate signalling molecule synthesized and secreted by activated B cells and plasma cells. We show that B cell-derived GABA promotes monocyte differentiation into anti-inflammatory macrophages that secrete interleukin-10 and inhibit CD8⁺ T cell killer function. In mice, B cell deficiency or B cell-specific inactivation of the GABA-generating enzyme GAD67 enhances anti-tumour responses. Our study reveals that, in addition to cytokines and membrane proteins, small metabolites derived from B-lineage cells have immunoregulatory functions, which may be pharmaceutical targets allowing fine-tuning of immune responses
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