| 1. |
- Binggeli, Christian, et al.
(författare)
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Lyman continuum leakage versus quenching with the James Webb Space Telescope : the spectral signatures of quenched star formation activity in reionization-epoch galaxies
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 479:1, s. 368-376
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we study the effects of a recent drop in star formation rate (SFR) on the spectra of epoch of reionization (EoR) galaxies, and the resulting degeneracy with the spectral features produced by extreme Lyman continuum leakage. In order to study these effects in the wavelength range relevant for the upcoming James Webb Space Telescope (JWST), we utilize synthetic spectra of simulated EoR galaxies from cosmological simulations together with synthetic spectra of partially quenched mock galaxies. We find that rapid declines in the SFR of EoR galaxies could seriously affect the applicability of methods that utilize the equivalent width of Balmer lines and the ultraviolet spectral slope to assess the escape fraction of EoR galaxies. In order to determine if the aforementioned degeneracy can be avoided by using the overall shape of the spectrum, we generate mock NIRCam observations and utilize a classification algorithm to identify galaxies that have undergone quenching. We find that while there are problematic cases, JWST/NIRCam or NIRSpec should be able to reliably identify galaxies with redshifts z similar to 7 that have experienced a significant decrease in the SFR (by a factor of 10-100) in the past 50-100 Myr with a success rate greater than or similar to 85 per cent. We also find that uncertainties in the dust-reddening effects on EoR galaxies significantly affect the performance of the results of the classification algorithm. We argue that studies that aim to characterize the dust extinction law most representative in the EoR would be extremely useful.
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| 2. |
- Blomberg, Niclas, 1986-
(författare)
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On Nuclear Norm Minimization in System Identification
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In system identification we model dynamical systems from measured data. This data-driven approach to modelling is useful since many real-world systems are difficult to model with physical principles. Hence, a need for system identification arises in many applications involving simulation, prediction, and model-based control.Some of the classical approaches to system identification can lead to numerically intractable or ill-posed optimization problems. As an alternative, it has recently been shown beneficial to use so called regularization techniques, which make the ill-posed problems ‘regular’. One type of regularization is to introduce a certain rank constraint. However, this in general still leads to a numerically intractable problem, since the rank function is non-convex. One possibility is then use a convex approximation of rank, which we will do here.The nuclear norm, i.e., the sum of the singular values, is a popular, convex surrogate of the rank function. This results in a heuristic that has been widely used in e.g. signal processing, machine learning, control, and system identification, since its introduction in 2001. The nuclear norm heuristic introduces a regularization parameter which governs the trade-off between model fit and model complexity. The parameter is difficult to tune, and thecurrent thesis revolves around this issue.In this thesis, we first propose a choice of the regularization parameter based on the statistical properties of fictitious validation data. This can be used to avoid computationally costly techniques such as cross-validation, where the problem is solved multiple times to find a suitable parameter value. The proposed choice can also be used as initialization to search methods for minimizing some criterion, e.g. a validation cost, over the parameter domain.Secondly, we study how the estimated system changes as a function of the parameter over its entire domain, which can be interpreted as a sensitivity analysis. For this we suggest an algorithm to compute a so called approximate regularization path with error guarantees, where the regularization path is the optimal solution as a function of the parameter. We are then able to guarantee the model fit, or, alternatively, the nuclear norm of the approximation, to deviate from the optimum by less than a pre-specified tolerance. Furthermore, we bound the l2-norm of the Hankel singular value approximation error, which means that in a certain subset of the parameter domain, we can guarantee the optimal Hankel singular values returned by the nuclear norm heuristic to not change more (in l2-norm) than a bounded, known quantity.Our contributions are demonstrated and evaluated by numerical examples using simulated and benchmark data.
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| 3. |
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| 4. |
- Dai, Liang, et al.
(författare)
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On the nuclear norm heuristic for a Hankel matrix completion problem
- 2015
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 51, s. 268-272
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Tidskriftsartikel (refereegranskat)abstract
- This note addresses the question if and why the nuclear norm heuristic can recover an impulse response generated by a stable single-real-pole system, if elements of the upper-triangle of the associated Hankel matrix are given. Since the setting is deterministic, theories based on stochastic assumptions for low-rank matrix recovery do not apply in the considered situation. A 'certificate' which guarantees the success of the matrix completion task is constructed by exploring the structural information of the hidden matrix. Experimental results and discussions regarding the nuclear norm heuristic applied to a more general setting are also given.
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| 5. |
- Giri, Sambit K., et al.
(författare)
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Constraining Lyman continuum escape using Machine Learning
- 2018
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Ingår i: Peering towards Cosmic Dawn. - : Cambridge University Press. - 9781107192461 ; , s. 254-258
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Konferensbidrag (refereegranskat)abstract
- The James Webb Space Telescope (JWST) will observe the rest-frame ultraviolet/optical spectra of galaxies from the epoch of reionization (EoR) in unprecedented detail. While escaping into the intergalactic medium, hydrogen-ionizing (Lyman continuum; LyC) photons from the galaxies will contribute to the bluer end of the UV slope and make nebular emission lines less prominent. We present a method to constrain leakage of the LyC photons using the spectra of high redshift (z greater than or similar to 6) galaxies. We simulate JWST/NIRSpec observations of galaxies at z = 6-9 by matching the fluxes of galaxies observed in the Frontier Fields observations of galaxy cluster MACS-J0416. Our method predicts the escape fraction f(esc) with a mean absolute error Delta f(esc) approximate to 0.14. The method also predicts the redshifts of the galaxies with an error approximate to 0.0003.
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| 6. |
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| 7. |
- Kotrschal, Alexander, et al.
(författare)
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Brain size does not impact shoaling dynamics in unfamiliar groups of guppies (Poecilia reticulata)
- 2018
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Ingår i: Behavioural Processes. - : Elsevier BV. - 0376-6357 .- 1872-8308. ; 147, s. 13-20
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Tidskriftsartikel (refereegranskat)abstract
- Collective movement is achieved when individuals adopt local rules to interact with their neighbours. How the brain processes information about neighbours' positions and movements may affect how individuals interact in groups. As brain size can determine such information processing it should impact collective animal movement. Here we investigate whether brain size affects the structure and organisation of newly forming fish shoals by quantifying the collective movement of guppies (Poecilia reticulata) from large- and small-brained selection lines, with known differences in learning and memory. We used automated tracking software to determine shoaling behaviour of single-sex groups of eight or two fish and found no evidence that brain size affected the speed, group size, or spatial and directional organisation of fish shoals. Our results suggest that brain size does not play an important role in how fish interact with each other in these types of moving groups of unfamiliar individuals. Based on these results, we propose that shoal dynamics are likely to be governed by relatively basic cognitive processes that do not differ in these brain size selected lines of guppies.
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| 8. |
- Kotrschal, Alexander, et al.
(författare)
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Evolution of brain region volumes during artificial selection for relative brain size
- 2017
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Ingår i: Evolution. - : Wiley. - 0014-3820 .- 1558-5646. ; 71:12, s. 2942-2951
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Tidskriftsartikel (refereegranskat)abstract
- The vertebrate brain shows an extremely conserved layout across taxa. Still, the relative sizes of separate brain regions vary markedly between species. One interesting pattern is that larger brains seem associated with increased relative sizes only of certain brain regions, for instance telencephalon and cerebellum. Till now, the evolutionary association between separate brain regions and overall brain size is based on comparative evidence and remains experimentally untested. Here, we test the evolutionary response of brain regions to directional selection on brain size in guppies (Poecilia reticulata) selected for large and small relative brain size. In these animals, artificial selection led to a fast response in relative brain size, while body size remained unchanged. We use microcomputer tomography to investigate how the volumes of 11 main brain regions respond to selection for larger versus smaller brains. We found no differences in relative brain region volumes between large- and small-brained animals and only minor sex-specific variation. Also, selection did not change allometric scaling between brain and brain region sizes. Our results suggest that brain regions respond similarly to strong directional selection on relative brain size, which indicates that brain anatomy variation in contemporary species most likely stem from direct selection on key regions.
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| 9. |
- Liu, Yang, et al.
(författare)
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APTER : Aggregated Prognosis Through Exponential Re-weighting
- 2019
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Ingår i: Computing and Combinatorics. - Cham : Springer Nature. - 9783030261764 - 9783030261757 ; , s. 425-436
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Konferensbidrag (refereegranskat)abstract
- This paper considers the task of learning how to make a prognosis of a patient based on his/her micro-array expression levels. The method is an application of the aggregation method as recently proposed in the literature on theoretical machine learning, and excels in its computational convenience and capability to deal with high-dimensional data. This paper gives a formal analysis of the method, yielding rates of convergence similar to what traditional techniques obtain, while it is shown to cope well with an exponentially large set of features. Those results are supported by numerical simulations on a range of publicly available survival-micro-array data sets. It is empirically found that the proposed technique combined with a recently proposed pre-processing technique gives excellent performances. All used software files and data sets are available on the authors' website http://user.it.uu.se/similar to liuya610/index.html
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| 10. |
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| 11. |
- Nygren, Johannes, et al.
(författare)
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A direct proof of the discrete-time multivariate circle and Tsypkin criteria
- 2016
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Ingår i: IEEE Transactions on Automatic Control. - 0018-9286 .- 1558-2523. ; 61:2, s. 544-549
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Tidskriftsartikel (refereegranskat)abstract
- This technical note presents a new proof of the circle criterion for multivariate, discrete-time systems with time-varying feedback nonlinearities. A new proof for the multivariate Tsypkin criterion for time-invariant monotonic feedback nonlinearities is derived as well. Both integrator- and non-integrator systems are considered. The proofs are direct in the sense that they do not resort to any existing result in systems theory, such as Lyapunov theory, passivity theory or the small-gain theorem. Instead, the proofs refer to the asymptotic properties of block-Toeplitz matrices. One major advantage of the new proof is that it elegantly handles integrator systems without resorting to loop transformation/pole shifting techniques. Additionally, less conservative stability bounds are derived by making stronger assumptions on the sector bound conditions on the feedback nonlinearities. In particular, it is exemplified how this technique relaxes stability conditions of (i) a model predictive control (MPC) rule and (ii) an integrator system.
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| 12. |
- Nygren, Johannes, et al.
(författare)
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A stability criterion for switching Lur'e systems with switching-path restrictions
- 2018
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 96, s. 337-341
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Tidskriftsartikel (refereegranskat)abstract
- This paper derives a stability criterion for switching Lur'e MIMO systems with time-varying nonlinearities, where switching is subject to given restrictions. These restrictions are encoded via the notion of a switching path, interpolating the criterion between the single-system case and arbitrary switching. The criterion for switching Lur'e systems is shown to boil down to a familiar LMI criterion for linear switching systems for the case the Lur'e systems have a linear feedback. The model of a switching Lur'e system is used to describe an MPC control scheme subject to loss of output readings. This example illustrates the practical and theoretical usefulness of the introduced switching path restrictions.
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| 13. |
- Nygren, Johannes, et al.
(författare)
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Conditions for input-output stability of discrete-time Luré systems with time-varying delays
- 2015
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Ingår i: Proc. 54th Conference on Decision and Control. - Piscataway, NJ : IEEE. - 9781479978861 ; , s. 7707-7714
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Konferensbidrag (refereegranskat)abstract
- This paper derives a stability condition for a type of Lur'e systems with time-varying delays and a feedback nonlinearity. The case of discrete-time systems is considered, consisting of a LTI, fed back though a time-varying static nonlinearity. There is an additional delay before or after this nonlinearity, which delays the signals by a positive, time-varying number of steps. Either the time-delay needs to be bounded by a constant (if the LTI system contains a single integrator) or its rate need to be bounded (in case the LTI system is stable). It turns out that, if the LTI has a single integrator with non-decreasing impulse response, the derived stability criterion coincides exactly with the circle criterion for the corresponding constant delay system. The technical proofs rely on direct manipulation of the involved signals, and do not make use of traditional tools as the small gain theorem, Lyapunov functions or passivity results.
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| 14. |
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| 15. |
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| 16. |
- Nygren, Johannes, 1984-
(författare)
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Input-Output Stability Analysis of Networked Control Systems
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main focus of the thesis is to derive stability criteria for networked control system (NCS) models featuring imperfections such as time-varying and constant delays, quantization, packet dropouts, and non-uniform sampling intervals. The main method of proof is based on matrix algebra, as opposed to methods using Lyapunov functions or integral quadratic constraints (IQC). This work puts a particular focus on handling systems with a single integrator. This framework is elaborated in different specific directions as motivated by practical realizations of NCSs, as well as through numerical examples. A novel proof of the discrete time multivariate circle criterion and the Tsypkin criterion for systems including a single integrator is presented, as well as a stability criterion for linear systems with a single integrator subject to variable sampling periods and sector-bounded nonlinear feedback. Four stability criteria for different classes of systems subject to packet loss and time-varying delay are given. Stability criteria for a closed loop system switching between a set of linear time-invariant systems (LTIs) are proved. This result is applied to a single-link NCS with feedback subject to packet loss. Finally, necessary and sufficient conditions for delay-independent stability of an LTI system subject to nonlinear feedback are derived.
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| 17. |
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| 18. |
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| 19. |
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| 20. |
- Spiegelberg, Jakob
(författare)
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Signal Processing Tools for Electron Microscopy
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The detection of weak signals in noisy data is a problem which occurs across various disciplines. Here, the signal of interest is the spectral signature of the electron magnetic chiral dichroism (EMCD) effect. In principle, EMCD allows for the measurement of local magnetic structures in the electron microscope, its spatial resolution, versatility and low hardware requirements giving it an eminent position among competing measurement techniques. However, experimental shortcomings as well as intrinsically low signal to noise ratio render its measurement challenging to the present day. This thesis explores how posterior data processing may aid the analysis of various data from the electron microscope. Following a brief introduction to different signals arising in the microscope and a yet briefer survey of the state of the art of EMCD measurements, noise removal strategies are presented. Afterwards, gears are shifted to discuss the separation of mixed signals into their physically meaningful source components based on their assumed mathematical characteristics, so called blind source separation (BSS). A data processing workflow for detecting weak signals in noisy spectra is derived from these considerations, ultimately culminating in several demonstrations of the extraction of EMCD signals. While the focus of the thesis does lie on data processing strategies for EMCD detection, the approaches presented here are similarly applicable in other situations. Related topics such as the general analysis of hyperspectral images using BSS methods or the fast analysis of large data sets are also discussed.
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| 21. |
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| 22. |
- Spiegelberg, Jakob, et al.
(författare)
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Unmixing hyperspectral data by using signal subspace sampling
- 2017
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Ingår i: Ultramicroscopy. - : ELSEVIER SCIENCE BV. - 0304-3991 .- 1879-2723. ; 182, s. 205-211
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Tidskriftsartikel (refereegranskat)abstract
- This paper demonstrates how Signal Subspace Sampling (SSS) is an effective pre-processing step for Non-negative Matrix Factorization (NMF) or Vertex Component Analysis (VCA). The approach allows to uniquely extract non-negative source signals which are orthogonal in at least one observation channel, respectively. It is thus well suited for processing hyperspectral images from X-ray microscopy, or other emission spectroscopies, into its non-negative source components. The key idea is to resample the given data so as to satisfy better the necessity and sufficiency conditions for the subsequent NMF or VCA. Results obtained both on an artificial simulation study as well as based on experimental data from electronmicroscopy are reported.
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| 23. |
- Stoica, Anca-Juliana, et al.
(författare)
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System components of a general theory of software engineering
- 2015
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Ingår i: Science of Computer Programming. - : Elsevier BV. - 0167-6423 .- 1872-7964. ; 101, s. 42-65
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Tidskriftsartikel (refereegranskat)abstract
- The contribution of this paper to a general theory of software engineering is twofold: it presents the model system concept, and it integrates the software engineering design process into a decision making theory and a value-based decision-under-risk process. The model system concept is defined as a collection of interconnected and consistent components that work together for defining, developing, and delivering a software system. This model system concept is used to represent the multiple facets of a software engineering project such as stakeholders and models related to domain/environment, success, decision, product, process, and property. The model system concept is derived from software development practices in the industry and academia. The theoretical decision framework acts as a central governance component for a given software engineering project. Applying this decision framework allows for effectively managing risks and uncertainties related to success in the project building stage. Especially, this puts the design process in an economic perspective, where concepts such as value-of-waiting, value-of-information and possible outcomes can be coped with explicitly. In practice, the decision framework allows for the optimal control of modern adaptive software development. In particular, one can use dynamic programming to find the optimal sequence of decisions to be made considering a defined time horizon. In this way we can relate our contribution to a theory of software engineering to the well-studied areas of automatic control, optimization, decision theory and Bayesian analysis.
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| 24. |
- Szorkovszky, Alex, et al.
(författare)
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An efficient method for sorting and quantifying individual social traits based on group-level behaviour
- 2017
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Ingår i: Methods in Ecology and Evolution. - 2041-210X. ; 8:12, s. 1735-1744
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Tidskriftsartikel (refereegranskat)abstract
- 1. In social contexts, animal behaviour is often studied in terms of group-level characteristics. One clear example of this is the collective motion of animals in decentralized structures, such as bird flocks and fish schools. A major goal of research is to identify how group-level behaviours are shaped by the traits of individuals within them. Few methods exist to make these connections. Individual assessment is often limited, forcing alternatives such as fitting agent-based models to experimental data. 2. We provide a systematic experimental method for sorting animals according to socially relevant traits, without assaying them or even tagging them individually. Instead, they are repeatedly subjected to behavioural assays in groups, between which the group memberships are rearranged, in order to test the effect of many different combinations of individuals on a group-level property or feature. We analyse this method using a general model for the group feature, and simulate a variety of specific cases to track how individuals are sorted in each case. 3. We find that in the case where the members of a group contribute equally to the group feature, the sorting procedure increases the between-group behavioural variation well above what is expected for groups randomly sampled from a population. For a wide class of group feature models, the individual phenotypes are efficiently sorted across the groups and thus become available for further analysis on how individual properties affect group behaviour. We also show that the experimental data can be used to estimate the individual-level repeatability of the underlying traits. 4. Our method allows experimenters to find repeatable variation in social behaviours that cannot be assessed in solitary individuals. Furthermore, experiments in animal behaviour often focus on comparisons between groups randomly sampled from a population. Increasing the behavioural variation between groups increases statistical power for testing whether a group feature is related to other properties of groups or to their phenotypic composition. Sorting according to socially relevant traits is also beneficial in artificial selection experiments, and for testing correlations with other traits. Overall, the method provides a useful tool to study how individual properties influence social behaviour.
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| 25. |
- Szorkovszky, Alex, et al.
(författare)
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Assortative interactions revealed by sorting of animal groups
- 2018
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Ingår i: Animal Behaviour. - : Elsevier BV. - 0003-3472 .- 1095-8282. ; 142, s. 165-179
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Tidskriftsartikel (refereegranskat)abstract
- Animals living in groups can show substantial variation in social traits and this affects their social organization. However, as the specific mechanisms driving this organization are difficult to identify in already organized groups typically found in the wild, the contribution of interindividual variation to group level behaviour remains enigmatic. Here, we present results of an experiment to create and compare groups that vary in social organization, and study how individual behaviour varies between these groups. We iteratively sorted individuals between groups of guppies, Poecilia reticulata, by ranking the groups according to their directional alignment and then mixing similar groups. Over the rounds of sorting the consistency of the group rankings increased, producing groups that varied significantly in key social behaviours such as collective activity and group cohesion. The repeatability of the underlying individual behaviour was then estimated by comparing the experimental data to simulations. At the level of basic locomotion, individuals in more coordinated groups displayed stronger interactions with the centre of the group, and weaker interactions with their nearest neighbours. We propose that this provides the basis for a passive phenotypic assortment mechanism that may explain the structures of social networks in the wild.
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| 26. |
- Yasini, Sholeh, et al.
(författare)
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High-dimensional online adaptive filtering
- 2017
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Konferensbidrag (refereegranskat)abstract
- While recent advances in online learning arising from the universal prediction perspective have led to algorithms with prediction performance guarantee, these techniques are not able to cope with high-dimensional data. This paper analyses a random projection gradient descent (RP-GD) algorithm which addresses this challenge and yields low theoretical regret bound and computationally efficient algorithm. It is shown that the performance of the algorithm converges to the performance of the best offline, computationally complex, high dimensional algorithm in hindsight.
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| 27. |
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| 28. |
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