Critical Behaviour of the Two Dimensional Step Model

We use finite--size scaling of Lee--Yang partition function zeroes to study the critical behaviour of the two dimensional step or sgn $O(2)$ model. We present evidence that, like the closely related $XY$--model, this has a phase transition from a disordered high temperature phase to a low temperature massless phase where the model remains critical. The critical parameters (including logarithmic corrections) are compatible with those of the $XY$--model indicating that both models belong to the same universality class.Comment: 6 pages latex, 3 postscript figures, compressed and uuencoded (revised remarks on Lee_Yang theorem, version to appear in Phys Rev B

The Modernization of Legal Lists

Worldwide, fatal traffic injuries and drowning deaths are important problems. The aim of this thesis was to investigate the cirumstances of fatal and non-fatal traffic injuries and drowning deaths in Sweden including analysis of the presence of alcohol and drugs, which are considered to be major risk factors for these events. Data where obtained from the database of National Board of Forensic Medicine. In the first study, we investigated 420 passenger deaths from 372 crashes during 1993-1996. There were 594 drivers involved. In total, 21% of the drivers at fault were alcohol positive compared to 2% of drivers not at fault (p<0.001) (Paper I). During 2004-2007, crashes involving 56 fatally and 144 non-fatally injured drivers were investigated in a prospective study from Northern Sweden (Paper II). The drivers were alcohol positive in 38% and 21%, respectively. Psychoactive drugs were found in 7% and 13%, respectively. Benzodiazepines, opiates and antidepressants were the most frequent drugs found in drivers. Illict drugs were found 9% and 4% respectively, with tetrahydrocannabinol being the most frequent of these drugs (Paper II). We investigated 5,125 drowning deaths in Sweden during 1992-2009 (Paper III). The incidence decreased on average by about 2% each year (p<0.001). Unintentional drowning was most common (50%). Alcohol was found in 44% of unintentional, 24% of intentional, and 45% of undetermined drowning deaths. Psychoactive substances were detected in 40% and benzodiazepines were the most common substance. Illicit drugs were detected in 10%. Of all drowning deaths, a significantly higher proportion females commited suicide compared with males (55% vs. 21%, p<0.001). Suicidal drowning deaths (n=129) in Northern Sweden were studied further in detail (Paper IV). of these, 53% had been hospitalized due to a psychiatric diagnosis within five years prior to the suicide. Affective and psychotic disorders were the most common psychiatric diagnoses. Almost one third had performed a previous suicide attempt. One fourth had committed suicide after less than one week of discharge from hospital. Alochol was found in 16% and psychoactive drugs in 62% of these cases, respectively.  In conclusion, alcohol and psychoactive drugs are commonly detected among injured drivers and drowning victims, and probably play a role in these events. Most of the individuals that tested positive for alcohol and high blood concentrations, indicating alochol dependence or abuse. This association warrants futher attention when planning future prevention.

Modeling Stable Matching Problems with Answer Set Programming

The Stable Marriage Problem (SMP) is a well-known matching problem first introduced and solved by Gale and Shapley (1962). Several variants and extensions to this problem have since been investigated to cover a wider set of applications. Each time a new variant is considered, however, a new algorithm needs to be developed and implemented. As an alternative, in this paper we propose an encoding of the SMP using Answer Set Programming (ASP). Our encoding can easily be extended and adapted to the needs of specific applications. As an illustration we show how stable matchings can be found when individuals may designate unacceptable partners and ties between preferences are allowed. Subsequently, we show how our ASP based encoding naturally allows us to select specific stable matchings which are optimal according to a given criterion. Each time, we can rely on generic and efficient off-the-shelf answer set solvers to find (optimal) stable matchings.Comment: 26 page

An analytically solvable model of probabilistic network dynamics

We present a simple model of network dynamics that can be solved analytically for uniform networks. We obtain the dynamics of response of the system to perturbations. The analytical solution is an excellent approximation for random networks. A comparison with the scale-free network, though qualitatively similar, shows the effect of distinct topology.Comment: 4 pages, 1 figur

Programming a hillslope water movement model on the MPP

A physically based numerical model was developed of heat and moisture flow within a hillslope on a parallel architecture computer, as a precursor to a model of a complete catchment. Moisture flow within a catchment includes evaporation, overland flow, flow in unsaturated soil, and flow in saturated soil. Because of the empirical evidence that moisture flow in unsaturated soil is mainly in the vertical direction, flow in the unsaturated zone can be modeled as a series of one dimensional columns. This initial version of the hillslope model includes evaporation and a single column of one dimensional unsaturated zone flow. This case has already been solved on an IBM 3081 computer and is now being applied to the massively parallel processor architecture so as to make the extension to the one dimensional case easier and to check the problems and benefits of using a parallel architecture machine

Stable marriage and roommates problems with restricted edges: complexity and approximability

In the Stable Marriage and Roommates problems, a set of agents is given, each of them having a strictly ordered preference list over some or all of the other agents. A matching is a set of disjoint pairs of mutually acceptable agents. If any two agents mutually prefer each other to their partner, then they block the matching, otherwise, the matching is said to be stable. We investigate the complexity of finding a solution satisfying additional constraints on restricted pairs of agents. Restricted pairs can be either forced or forbidden. A stable solution must contain all of the forced pairs, while it must contain none of the forbidden pairs. Dias et al. (2003) gave a polynomial-time algorithm to decide whether such a solution exists in the presence of restricted edges. If the answer is no, one might look for a solution close to optimal. Since optimality in this context means that the matching is stable and satisfies all constraints on restricted pairs, there are two ways of relaxing the constraints by permitting a solution to: (1) be blocked by as few as possible pairs, or (2) violate as few as possible constraints n restricted pairs. Our main theorems prove that for the (bipartite) Stable Marriage problem, case (1) leads to View the MathML source-hardness and inapproximability results, whilst case (2) can be solved in polynomial time. For non-bipartite Stable Roommates instances, case (2) yields an View the MathML source-hard but (under some cardinality assumptions) 2-approximable problem. In the case of View the MathML source-hard problems, we also discuss polynomially solvable special cases, arising from restrictions on the lengths of the preference lists, or upper bounds on the numbers of restricted pairs