Luonnontieteet / Natural sciences
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Helsingin yliopiston tutkijoiden julkaisemia artikkeleja.
Helsingin yliopiston tutkijat voivat rinnakkaistallentaa tutkimusjulkaisujansa HELDAan liittämällä kokotekstin julkaisuun TUHAT-tutkimustietojärjestelmässä. (Toimintaohje tutkijalle)
Articles published by researchers at the University of Helsinki
Recent Submissions
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(Applied Physics Letters, 2009)We increase the effective stiffness of optical tweezers by position clamping a polystyrene bead with a predictive feedback control algorithm. This algorithm mitigates the effect of feedback loop delay. Hence, higher gain than with proportional control can be employed, which results in higher effective trap stiffness, without trap instability. In experiments (initial trap stiffness 0.056 pN/nm with a 1.78 μm diameter polystyrene bead) predictive control increased the effective trap stiffness by 55% relative to proportional control. We also derive theoretical expressions for the power spectra of the bead position controlled by our algorithm.
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(Elsevier, 2008)Environmental Science and Policy 11To reduce GHG emissions, the EU27 countries committed themselves in 2007 to reduce emissions from 1990 levels by 20% by 2020. In January 2008, the EU commission gave the first country-specific proposals to reduce emissions in sectors outside the EU emission trading system (non-ETS). In this study, we looked at several ways of sharing emission reductions in the non-ETS sector. We considered population and economic growth as significant drivers of the development of emissions. In particular, we analyzed development in GHG intensity of economies. Reduction requirements vary greatly among countries depending on the principle of effort sharing. The results of our calculations can be perceived as examples of how effort sharing between the EU Member States could look like when certain assumptions are made. Generally they illustrate the sensitivity of the results to data used, assumptions made, and method applied. The main strength of simple top-down approaches is transparency. A major weakness is a very limited ability to consider national circumstances. Political negotiations are ultimately crucial; an analysis like this provides material for negotiations and makes a contribution to solving the effort-sharing problem. As future development is partly unpredictable, implementation of some kind of subsequent adjustment could be considered during the process.
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(2009)We present a fast space-efficient algorithm for constructing compressed suffix arrays (CSA). The algorithm requires O(n log n) time in the worst case, and only O(n) bits of extra space in addition to the CSA. As the basic step, we describe an algorithm for merging two CSAs. We show that the construction algorithm can be parallelized in a symmetric multiprocessor system, and discuss the possibility of a distributed implementation. We also describe a parallel implementation of the algorithm, capable of indexing several gigabytes per hour.
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(Centralförbundet för Fiskerihushållning, 2002)Fiskeritidskrift för Finland
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(Centralförbundet för Fiskerihushållning, 1994)Fiskeritidskrift för Finland
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(Centralförbundet för Fiskerihushållning, 2002)Fiskeritidskrift för Finland
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(Ympäristö- ja terveysalan tekniset / Ympäristönsuojeluviranhaltijat, 2008)Ympäristö ja Terveys
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(Economist Newspaper Ltd., 2009)The EconomistSIR – In looking for cost-efficient CCS, please step up and walk to your window, where you may see a tree. The evolution of woody plants has solved the problem of capture (photosynthesis) and storage (formation of durable cells) at minimal cost. After what is called “forest transition”, woody resources of a country cease to shrink and start to expand. Forest transition implies a shift of the landscape from a carbon source to a carbon sink, thus marking the onset of organic, cheap CCS. Alexander Mather of the University of Aberdeen predicted in 1992 that forest transition is the likely future of tropical countries, too. Since then, however, biofuel clearings and other pressures have created new concerns. Organic CCS will again become an issue as climate negotiators reconvene to consider a post-Kyoto treaty in Copenhagen in December this year. Pekka Kauppi Professor of environmental science and policy University of Helsinki Helsinki
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(Metsähallitus, 2009)metsä.fi
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(Old City Publishing, 2008)We study the algorithmic problem of coordinating transmissions in a wireless network where radio interference constrains concurrent transmissions on wireless links. We focus on pairwise conflicts between the links; these can be described as a conflict graph. Associated with the conflict graph are two fundamental network coordination tasks: (a) finding a nonconflicting set of links with the maximum total weight, and (b) finding a link schedule with the minimum total length. Our work shows that two assumptions on the geometric structure of conflict graphs suffice to achieve polynomial-time constant-factor approximations: (i) bounded density of devices, and (ii) bounded range of interference. We also show that these assumptions are not sufficient to obtain a polynomial-time approximation scheme (PTAS) for either coordination task. There exists a PTAS if we make an additional assumption: (iii) bounded range of radio transmissions.
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(Elsevier, 2009)We present a local algorithm (constant-time distributed algorithm) for finding a 3-approximate vertex cover in bounded-degree graphs. The algorithm is deterministic, and no auxiliary information besides port numbering is required.
