Efficient generation of random derangements with the expected distribution of cycle lengths

We show how to generate random derangements efficiently by two different techniques: random restricted transpositions and sequential importance sampling. The algorithm employing restricted transpositions can also be used to generate random fixed-point-free involutions only, a.k.a. random perfect matchings on the complete graph. Our data indicate that the algorithms generate random samples with the expected distribution of cycle lengths, which we derive, and for relatively small samples, which can actually be very large in absolute numbers, we argue that they generate samples indistinguishable from the uniform distribution. Both algorithms are simple to understand and implement and possess a performance comparable to or better than those of currently known methods. Simulations suggest that the mixing time of the algorithm based on random restricted transpositions (in the total variance distance with respect to the distribution of cycle lengths) is $O(n^{a}\log{n}^{2})$ with $a \simeq \frac{1}{2}$ and $n$ the length of the derangement. We prove that the sequential importance sampling algorithm generates random derangements in $O(n)$ time with probability $O(1/n)$ of failing.Comment: This version corrected and updated; 14 pages, 2 algorithms, 2 tables, 4 figure

Two-stream instability in quasi-one-dimensional Bose-Einstein condensates

We apply a kinetic model to predict the existence of an instability mechanism in elongated Bose-Einstein condensates. Our kinetic description, based on the Wigner formalism, is employed to highlight the existence of unstable Bogoliubov waves that may be excited in the counterpropagation configuration. We identify a dimensionless parameter, the Mach number at T=0, that tunes different regimes of stability. We also estimate the magnitude of the main parameters at which two-stream instability is expected to be observed under typical experimental conditions

Management Of Fishing Of The Broadband Anchovy (anchoviella Lepidentostole) (fowler, 1911), In South São Paulo State, Brazil.

The broadband anchovy (Anchoviella lepidentostole) is one of the most important fishery resources for economic and social development of Iguape City and for the whole of the south coast of São Paulo state. Nowadays this activity involves over 2,500 fishermen. This paper aims to evaluate its fishing, and discuss how the activity is managed. In order to conduct this study, production data and catch per unit effort (CPUE) data from the 1998-2000 period have been collected, and the characterisation of the activity has also been done. A decline in broadband anchovy abundance had been observed in the region up until 2004, when a new regulation was implemented, in an attempt to recover the resource, with the establishment of closed season fishing (defeso) during the catch season, and adjustments in the employed fishing gear. The results have shown a recovery of the abundance of broadband anchovy throughout the 2005-2010 period, showing thus far that the measures taken for the maintenance of the resource have been successful. The activity has been managed through discussions involving people from the fishing sector in order to determine the controversial actions to be taken, that are hard to enforce. In order to achieve a more satisfactory resource management, it is necessary to keep the participatory process, with adaptive co-management, based on discussion among all the sectors involved (from government and society), and constant monitoring of the activity.73691-

Nonlinear excitation of zonal flows by rossby wave turbulence

We apply the wave-kinetic approach to study nonlinearly coupled Rossby wave-zonal flow fluid turbulence in a two-dimensional rotating fluid. Specifically, we consider for the first time nonlinear excitations of zonal flows by a broad spectrum of Rossby wave turbulence. Short-wavelength Rossby waves are described here as a fluid of quasi-particles, and are referred to as the 'Rossbyons'. It is shown that Reynolds stresses of Rossbyons can generate large-scale zonal flows. The result should be useful in understanding the origin of large-scale planetary and near-Earth atmospheric circulations. It also provides an example of a turbulent wave background driving a coherent structure