Reducing Reparameterization Gradient Variance

Optimization with noisy gradients has become ubiquitous in statistics and machine learning. Reparameterization gradients, or gradient estimates computed via the "reparameterization trick," represent a class of noisy gradients often used in Monte Carlo variational inference (MCVI). However, when these gradient estimators are too noisy, the optimization procedure can be slow or fail to converge. One way to reduce noise is to use more samples for the gradient estimate, but this can be computationally expensive. Instead, we view the noisy gradient as a random variable, and form an inexpensive approximation of the generating procedure for the gradient sample. This approximation has high correlation with the noisy gradient by construction, making it a useful control variate for variance reduction. We demonstrate our approach on non-conjugate multi-level hierarchical models and a Bayesian neural net where we observed gradient variance reductions of multiple orders of magnitude (20-2,000x)

Egg parasitoid exploitation of plant volatiles induced by single or concurrent attack of a zoophytophagous predator and an invasive phytophagous pest

Zoophytophagous insect predators can induce physiological responses in plants by activating defence signalling pathways, but whether plants can respond to facultative phytophagy by recruiting natural enemies remains to be investigated. In Y-tube olfactometer bioassays, using a system including a Vicia faba plant, the zoophytophagous predator Podisus maculiventris and the egg parasitoid Telenomus podisi, we first demonstrated that T. podisi females are attracted by broad bean plants damaged by feeding activity of P. maculiventris and on which host egg masses had been laid, while they are not attracted by undamaged plants or plants damaged by feeding activity alone. In a second experiment, we evaluated the impact of the invasive phytophagous pest Halyomorpha halys on this plant volatile-mediated tritrophic communication. Results showed that the invasive herbivorous adults do not induce plants to recruit the native egg parasitoid, but they can disrupt the local infochemical network. In fact, T. podisi females are not attracted by volatiles emitted by plants damaged by H. halys feeding alone or combined with oviposition activity, nor are they attracted by plants concurrently infested by P. maculiventris and H. halys, indicating the specificity in the parasitoid response and the ability of the invasive herbivore in interrupting the semiochemical communication between plants and native egg parasitoids. To the best of our knowledge, this is the first study showing that zoophytophagous predator attacks induce indirect plant defences similarly to those defence strategies adopted by plants as a consequence of single or concurrent infestations of herbivorous insects

A new approach in the use of SIT in determining the dependence on ionic strength of activity coefficients. Application to some chloride salts of interest in the speciation of natural fluids

AbstractThis paper describes a modified version of the SIT (Specific ion Interaction Theory) method and its use in determining the dependence on ionic strength of activity coefficients. In the new approach the interaction coefficients (e) are not constant but depend on ionic strength (I /mol kg-1) according to the simple relationship:e = e∞+ (e0 - e∞) / (l + 1)where e0 and are true constants for I→ 0 and l→ ∞, respectively. To check the two parameter SIT equation, we calculated e0 and for the activity coefficients of HCl, LiCl, NaCl, KCl, MgCl2, CaCl2 and SrCl2, in a wide ionic strength range (0.1 ≤ l/mol kg-1 ≤ 4.5, for KCl; 0.1 ≤ l/mol kg-1 ≤ 6, for HCl, LiCl, NaCl; 0.3 ≤ l/mol kg-1 ≤ 12, for SrCl2; 0.3 ≤ l/mol kg-1 ≤ 15, for MgCl2; 0.3 ≤ l/mol kg-1 ≤ 18, for CaCl2). Results show that the γ values calculated using this approach fit quite well over the whole I-range for all the electrolytes considered. Comparison is made with the analogous one parameter SIT equation. The temperature coefficients of inter..

Effective description of the short-time dynamics in open quantum systems

We address the dynamics of a bosonic system coupled to either a bosonic or a magnetic environment, and derive a set of sufficient conditions that allow one to describe the dynamics in terms of the effective interaction with a classical fluctuating field. We find that for short interaction times the dynamics of the open system is described by a Gaussian noise map for several different interaction models and independently on the temperature of the environment. In order to go beyond a qualitative understanding of the origin and physical meaning of the above short-time constraint, we take a general viewpoint and, based on an algebraic approach, suggest that any quantum environment can be described by classical fields whenever global symmetries lead to the definition of environmental operators that remain well defined when increasing the size, i.e. the number of dynamical variables, of the environment. In the case of the bosonic environment this statement is exactly demonstrated via a constructive procedure that explicitly shows why a large number of environmental dynamical variables and, necessarily, global symmetries, entail the set of conditions derived in the first part of the work.Comment: 9 pages, close to published versio