Archive for Leipzig

Nature snapshots

Posted in Books, Kids, Mountains, pictures, Travel with tags , , , , , , , , , , , , , , , , , , , , , , , on September 19, 2024 by xi'an

Some quick breakfast reads from the 22 August issue of Nature, beyond the nice pun in the cover title (which reads like Lonely Planet at first glance!) capturing a rare blooming of plants in the drylands of the Judaean Desert in 2015, discussing the higher diversity of plants in dry environments,

  • a tribune about the declining number of junior researchers in South Korea (with a supplementary Nature Index on its remarkable achievements), due to lower birth rates. The government is trying to make postdoc positions more attractive, to attract international students and postdocs. It has also signed with the EU to become an associate member of the Horizon Europe program, hence potentially benefitting of ERC grants.
  • looking for the hottest temperature compatible with humans, alas soon in a theatre near you… (With a heatwave chamber that was driven from Brisbane to Sydney!) I did not understand, though, how the limit of 31⁰C as the minimal temperature at which a healthy, young person would die after six hours of exposure. But agreed with the immediate benefits of skin-wetting, which I use almost constantly on hot days and nights.
  • the false good idea of wood pellets, reminding me of a freezing August in Christchurch back in 2005!, as pellets (and other biomass energy) generate more carbon than coal, favour deforestation, impacts the health of communities surrounding facilities, and takes decades to reach neutral outcomes.
  • another possibly false good idea, floating and sustainable settlements in coastal regions threatened by sea rises. Not only floating cities are expensive to build, but they are more exposed to extreme weather events, compete with the preservation of wetlands and mangroves, and cannot function without adapted infrastructure, from water and sewage treatment to means of transportation, and enough nearby services.
  • the ERROR project that pays for spotting mistakes in published papers, developed by the Universities of Bern and Leipzig. ERROR stands for Estimating the Reliability and Robustness of Research. At 2,500 Swiss francs per paper, this is hardly sustainable…
  • the problem of artificial neural networks losing plasticity in continual-learning settings and a potential solution via back-propagation.

 

gradient importance sampling

Posted in Books, pictures, Statistics, University life with tags , , , , , , on July 30, 2015 by xi'an

from my office, La Défense & Bois de Boulogne, Paris, May 15, 2012Ingmar Schuster, who visited Paris-Dauphine last Spring (and is soon to return here as a postdoc funded by Fondation des Sciences Mathématiques de Paris) has arXived last week a paper on gradient importance sampling. In this paper, he builds a sequential importance sampling (or population Monte Carlo) algorithm that exploits the additional information contained in the gradient of the target. The proposal or importance function being essentially the MALA move as its proposal, mixed across the elements of the previous population. When compared with our original PMC mixture of random walk proposals found in e.g. this paper, each term in the mixture thus involves an extra gradient, with a scale factor that decreases to zero as 1/t√t. Ingmar compares his proposal with an adaptive Metropolis, an adaptive MALTa and an HM algorithms, for two mixture distributions and the banana target of Haario et al. (1999) we also used in our paper. As well as a logistic regression. In each case, he finds both a smaller squared error and a smaller bias for the same computing time (evaluated as the number of likelihood evaluations). While we discussed this scheme when he visited, I remain intrigued as to why it works so well when compared with the other solutions. One possible explanation is that the use of the gradient drift is more efficient on a population of particles than on a single Markov chain, provided the population covers all modes of importance on the target surface: the “fatal” attraction of the local model is then much less of an issue…