On the non-convergence of the Wang-Landau algorithms with multiple random walkers

This paper discusses some convergence properties in the entropic sampling Monte Carlo methods with multiple random walkers, particularly in the Wang-Landau (WL) and $1/t$ algorithms. The classical algorithms are modified by the use of $m$ independent random walkers in the energy landscape to calculate the density of states (DOS). The Ising model is used to show the convergence properties in the calculation of the DOS, as well as the critical temperature, while the calculation of the number $\pi$ by multiple dimensional integration is used in the continuum approximation. In each case, the error is obtained separately for each walker at a fixed time, $t$; then, the average over $m$ walkers is performed. It is observed that the error goes as $1/\sqrt{m}$. However, if the number of walkers increases above a certain critical value $m>m_x$, the error reaches a constant value (i.e. it saturates). This occurs for both algorithms; however, it is shown that for a given system, the $1/t$ algorithm is more efficient and accurate than the similar version of the WL algorithm. It follows that it makes no sense to increase the number of walkers above a critical value $m_x$, since it does not reduces the error in the calculation. Therefore, the number of walkers does not guarantee convergence.Comment: 10 pages, 12 figures, Regular Articl

Optical polarimetric monitoring of the type II-plateau SN 2005af

Aims. Core-collapse supernovae may show significant polarization that implies non-spherically symmetric explosions. We observed the type II-plateau SN 2005af using optical polarimetry in order to verify whether any asphericity is present in the supernova temporal evolution. Methods. We used the IAGPOL imaging polarimeter to obtain optical linear polarization measurements in R (five epochs) and V (one epoch) broadbands. Interstellar polarization was estimated from the field stars in the CCD frames. The optical polarimetric monitoring began around one month after the explosion and lasted ~30 days, between the plateau and the early nebular phase. Results. The weighted mean observed polarization in R band was [1.89 +/- 0.03]% at position angle (PA) 54 deg. After foreground subtraction, the level of the average intrinsic polarization for SN 2005af was ~0.5% with a slight enhancement during the plateau phase and a decline at early nebular phase. A rotation in PA on a time scale of days was also observed. The polarimetric evolution of SN 2005af in the observed epochs is consistent with an overall asphericity of ~20% and an inclination of ~30 deg. Evidence for a more complex, evolving asphericity, possibly involving clumps in the SN 2005af envelope, is found.Comment: 6 pages, 5 figures, to be published A&

Larval development of Sesarma catenata Ortmann (Brachyura, Grapsidae, Sesarminae) reared in the laboratory

Sesarma catenata larvae were raised in the laboratory and the development stages described in detail. larval development comprises four zoea and one megalopa stage. Morphological features of the first zoea and megalopa stages of Sesarma catenata are compared with those of other Sesarminae species described in the literature. The genera Sesarma, Metasesarma, and Bresedium appear to be closely related taxa within the subfamily. The diagnostic value of some features of larval morphology is briefly discussed.S. Afr. J. Zool.1987, 22(3

Tidal exchange of larvae of Sesarma catenata (Decapoda, Brachyura) in the Swartkops estuary, South Africa

The tidal exchange of larvae of the salt-marsh grapsid crab Sesarma catenata was studied in the Swartkops estuary, a tidally driven, shallow estuary in Algoa Bay, South Africa. Plankton samples were collected bimonlhly during spring and neap tides from October to March at the tidal inlet. Samples were collected hourly for 25 h in February, and for 13 h in all other months. Hourly estimates of water flux through the tidal inlet of lhe esluary were calculaled using a generalized one-dimensional hydrodynamic model calibrated for lhe Swartkops estuary. Water flux estimates were used to calculate larval flux through the tidal inlet, Zoea I larvae of Sesarma catenata were found in all sampling sessions, with highest abundance during nocturnal ebb tides. Two seasonal peaks of abundance were detected in November and February-March. These coincided with recorded peaks of percentage abundance of gravid females in the adult population. Megalopa larvae were recorded mainly during flood tides from January to March. More than 70 % of all Sesarma catenata larvae moving through the tidal inlet of the Swartkops estuary in either direction were exported in any given sampling session. There was net export of zoea l larvae in all cases but one, and net import of megalopa larvae in all cases. The possibility that Sesarma catenata larvae may become entrapped in lhe surf zone circulation system throughout development, as a mechanism to avoid offshore dispersal, is discussed

The zoeal development of Sesarma eulimene de Man (Decapoda, Brachyura, Grapsidae), and identification of larvae of the genus Sesarma in South African waters

The five zoea larval stages of the estuarinecrab Sesarma eulimene de Man 1897 (Brachyura, Grapsidae) are described in detail from specimens reared in the laboratory. The morphology of larval Grapsidae is briefly discussed and an updated summary of larval diagnostic characters in the genus Sesarma is given as an aid to identifying specimens in plankton samples from Soulh African waters

The larval development of the red mangrove crab Sesarma meinerti de man (Brachyura: Grapsidae) reared in the laborator

The larval stages of the red mangrove crab Sesarma meinerti de Man were reared in the laboratory. Larval development consists of five zoeal stages and one megalopa. Zoeal development lasts an average of 25 days at 25°C. The external morphology of larvae is described in detail and their relationship with larvae of congeneric species is briefly discussed