| 1. |
- Lindström, Torsten, 1965-, et al.
(författare)
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Destabilization, Stabilization, and Multiple Attractors in Saturated Mixotrophic Environments
- 2020
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Ingår i: SIAM Journal on Applied Mathematics. - United States : Society for Industrial and Applied Mathematics. - 0036-1399 .- 1095-712X. ; 80:6, s. 2338-2364
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Tidskriftsartikel (refereegranskat)abstract
- The ability of mixotrophs to combine phototrophy and phagotrophy is now well recognized and found to have important implications for ecosystem dynamics. In this paper, we examine the dynamical consequences of the invasion of mixotrophs in a system that is a limiting case of the chemostat. The model is a hybrid of a competition model describing the competition between autotroph and mixotroph populations for a limiting resource, and a predator--prey-type model describing the interaction between autotroph and herbivore populations. Our results show that mixotrophs are able to invade in both autotrophic environments and environments described by interactions between autotrophs and herbivores. The interaction between autotrophs and herbivores might be in equilibrium or cycle. We find that invading mixotrophs have the ability to both stabilize and destabilize autotroph-herbivore dynamics depending on the competitive ability of mixotrophs. The invasion of mixotrophs can also result in multiple attractors.
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| 2. |
- Amouzad Mahdiraji, Saeid, et al.
(författare)
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A Micro-Level Simulation Model for Analyzing the Use of MSUs in Southern Sweden
- 2021
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Ingår i: Procedia Computer Science : 12th International Conference on Emerging Ubiquitous Systems and Pervasive Networks, EUSPN 2021 / 11th International Conference on Current and Future Trends of Information and Communication Technologies in Healthcare, ICTH 2021 - 12th International Conference on Emerging Ubiquitous Systems and Pervasive Networks, EUSPN 2021 / 11th International Conference on Current and Future Trends of Information and Communication Technologies in Healthcare, ICTH 2021. - : Elsevier BV. ; 198, s. 132-139
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Konferensbidrag (refereegranskat)abstract
- A mobile stroke unit (MSU) is a special type of ambulance, where stroke patients can be diagnosed and provided intravenous treatment, hence allowing to cut down the time to treatment for stroke patients. We present a discrete event simulation (DES) model to study the potential benefits of using MSUs in the southern health care region of Sweden (SHR). We included the activities and actions used in the SHR for stroke patient transportation as events in the DES model, and we generated a synthetic set of stroke patients as input for the simulation model. In a scenario study, we compared two scenarios, including three MSUs each, with the current situation, having only regular ambulances. We also performed a sensitivity analysis to further evaluate the presented DES model. For both MSU scenarios, our simulation results indicate that the average time to treatment is expected to decrease for the whole region and for each municipality of SHR. For example, the average time to treatment in the SHR is reduced from 1.31h in the baseline scenario to 1.20h and 1.23h for the two MSU scenarios. In addition, the share of stroke patients who are expected to receive treatment within one hour is increased by a factor of about 3 for both MSU scenarios.
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| 3. |
- Amouzad Mahdiraji, Saeid, et al.
(författare)
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An Optimization Model for the Placement of Mobile Stroke Units
- 2024
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Ingår i: Advanced Research in Technologies, Information, Innovation and Sustainability - 3rd International Conference, ARTIIS 2023, Proceedings. - : Springer. - 1865-0929 .- 1865-0937. - 9783031488573 - 9783031488580 ; 1935 CCIS, s. 297-310
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Konferensbidrag (refereegranskat)abstract
- Mobile Stroke Units (MSUs) are specialized ambulances that can diagnose and treat stroke patients; hence, reducing the time to treatment for stroke patients. Optimal placement of MSUs in a geographic region enables to maximize access to treatment for stroke patients. We contribute a mathematical model to optimally place MSUs in a geographic region. The objective function of the model takes the tradeoff perspective, balancing between the efficiency and equity perspectives for the MSU placement. Solving the optimization problem enables to optimize the placement of MSUs for the chosen tradeoff between the efficiency and equity perspectives. We applied the model to the Blekinge and Kronoberg counties of Sweden to illustrate the applicability of our model. The experimental findings show both the correctness of the suggested model and the benefits of placing MSUs in the considered regions.
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| 4. |
- Areitioaurtena, Maialen, et al.
(författare)
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A semi-analytical coupled simulation approach for induction heating
- 2021
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Ingår i: Advanced Modeling and Simulation in Engineering Sciences. - : Springer Science and Business Media LLC. - 2213-7467. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- The numerical simulation of the induction heating process can be computationally expensive, especially if ferromagnetic materials are studied. There are several analytical models that describe the electromagnetic phenomena. However, these are very limited by the geometry of the coil and the workpiece. Thus, the usual method for computing more complex systems is to use the finite element method to solve the set of equations in the multiphysical system, but this easily becomes very time consuming. This paper deals with the problem of solving a coupled electromagnetic - thermal problem with higher computational efficiency. For this purpose, a semi-analytical modeling strategy is proposed, that is based on an initial finite element computation, followed by the use of analytical electromagnetic equations to solve the coupled electromagnetic-thermal problem. The usage of the simplified model is restricted to simple geometrical features such as flat or curved surfaces with great curvature to skin depth ratio. Numerical and experimental validation of the model show an average error between 0.9% and 4.1% in the prediction of the temperature evolution, reaching a greater accuracy than other analyzed commercial softwares. A 3D case of a double-row large size ball bearing is also presented, fully validating the proposed approach in terms of computational time and accuracy for complex industrial cases.
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| 5. |
- Brodén, Björn, et al.
(författare)
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Guarding Lines and 2-Link Polygons is APX-hard
- 2001
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We prove that the minimum line covering problem and the minimum guard covering problem restricted to 2-link polygons are APX-hard.
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| 6. |
- Brundin, Ethel, et al.
(författare)
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Triple Helix Networks in a Multicultural Context : Triggers and Barriers for Fostering Growth and Sustainability
- 2008
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Ingår i: Journal of Developmental Entrepreneurship. - 1084-9467. ; 13:1, s. 77-98
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Tidskriftsartikel (refereegranskat)abstract
- This article deals with Triple Helix (university, industry and government co-operation) from an institutional theory perspective. The empirical context is the Western Cape Region in South Africa and the focus is entrepreneurship development. The purpose is twofold: first, the existing Triple Helix model is adapted to the South African context; and second, facilities and impediments for working according to Triple Helix in South Africa are identified. The empirical material consists of a survey and three longitudinal case studies illustrating the degree of co-operation between the three parties. The article contributes to knowledge about how the Triple Helix model works on a regional level in a developing country. The study draws the following conclusions: when co-operation is to be identified between the three actors, only two of the three are involved; one missing link in the Triple Helix model is the focus on the entrepreneur; co-operation between the three parties are incidental rather than planned and there is lack of structure. In turn, some of these conclusions may be an effect of institutional changes on a national level. For a normative legacy, the article proposes a set of suggestions for incorporating all relevant parties on a practical level.
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| 7. |
- Hammar, Mikael, et al.
(författare)
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Parallel Searching on m Rays
- 2001
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Ingår i: Computational geometry. - : Elsevier. - 0925-7721 .- 1879-081X. ; 18:3, s. 125-139
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Tidskriftsartikel (refereegranskat)abstract
- We investigate parallel searching on $m$ concurrent rays. We assume that a target $t$ is located somewhere on one of the rays; we are given a group of $m$ point robots each of which has to reach $t$. Furthermore, we assume that the robots have no way of communicating over distance. Given a strategy $S$ we are interested in the competitive ratio defined as the ratio of the time needed by the robots to reach $t$ using $S$ and the time needed to reach $t$ if the location of $t$ is known in advance. If a lower bound on the distance to the target is known, then there is a simple strategy which achieves a competitive ratio of~9 --- independent of $m$. We show that 9 is a lower bound on the competitive ratio for two large classes of strategies if $m\geq 2$. If the minimum distance to the target is not known in advance, we show a lower bound on the competitive ratio of $1 + 2 (k + 1)^{k + 1} / k^k$ where $k = \left\lceil\log m\right\rceil$ where $\log$ is used to denote the base 2 logarithm. We also give a strategy that obtains this ratio.
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| 8. |
- Hansson, Mattias, et al.
(författare)
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Bayesian Probability Maps For Evaluation Of Cardiac Ultrasound Data
- 2009
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Ingår i: Proceedins of PMMIA 2009. ; , s. 45-56
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we propose a Bayesian approach for describing the position distribution of the endocardium in cardiac ultrasound image sequences. The problem is formulated using a latent variable model, which represents the inside and outside of the endocardium, for which the posterior density is estimated. As the Rayleigh distribution has been previously shown to be a suitable model for blood and tissue in cardiac ultrasound image, we start our construction by assuming a Rayleigh mixture model and estimate its parameters by expectation maximization. The model is refined by incorporating priors for spatial and temporal smoothness, in the form of total variation, preferred shapes and position, by using the principal components and location distribution of manually segmented training shapes. The posterior density is sampled by a Gibbs method to estimate the expected latent variable image which we call the Bayesian Probability Map, since it describes the probability of pixels being classified as either heart tissue or within the endocardium. Our experiments showed promising results indicating the usefulness of the Bayesian Probability Maps for the clinician since, instead of producing a single segmenting curve, it highlights the uncertain areas and suggests possible segmentations.
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| 9. |
- Hansson, Mattias, et al.
(författare)
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Convex spatio-temporal segmentation of the endocardium in ultrasound data using distribution and shape priors
- 2011
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Ingår i: 2011 IEEE International Symposium on Biomedical Imaging: From Nano to Macro. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781424441273
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Konferensbidrag (refereegranskat)abstract
- We present a convex variational active contour model with shape priors, for spatio-temporal segmentation of the endocardium in 2D B-mode ultrasound sequences, which can be solved by Continuous Cuts. A four component (signal dropout, echocardiographic artifacts, blood and tissue) Rayleigh mixture model is proposed for modeling the inside and outside of the endocardium. The parameters of the mixture model are determined by Expectation Maximization, for the sequence. Annotated data is used to provide prior data, by which prior distributions for the inside and outside of the endocardium are constructed. Segmentation is then achieved by minimizing the Hellinger distance between prior and estimated distributions, under the constraints of a statistical shape prior built from principal eigenvectors of the annotated data. Since our model is convex, we can employ a fast optimization method: the Split-Bregman algorithm. Promising segmentation results and quantitative measures are provided.
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| 10. |
- Hansson, Mattias, et al.
(författare)
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Rayleigh segmentation of the endocardium in ultrasound images
- 2008
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Ingår i: Proceedings of the 19th International Conference on Pattern Recognition (ICPR). - : IEEE. ; , s. 1-4
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we present the coupled active contours (CAC) model, which is applied to segmentation of the endocardium in ultrasonic images assuming Rayleigh distributed intensities. Comparative experiments, both real and synthetic, with a standard prior model are presented. In the CAC model the prior acts, by affine transformation, on the same image information as the active contour, in addition to the traditional interaction between prior and active contour. By this higher convergence rate and robustness, w.r.t artifacts and poor initialization, is achieved.
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