97 research outputs found
Chiral effective field theories of the strong interactions
Effective field theories of the strong interactions based on the approximate
chiral symmetry of QCD provide a model-independent approach to low-energy
hadron physics. We give a brief introduction to mesonic and baryonic chiral
perturbation theory and discuss a number of applications. We also consider the
effective field theory including vector and axial-vector mesons.Comment: 22 pages, 9 figures, proceedings of "Many-Body Structure of Strongly
Interacting Systems", Mainz, Germany, Feb. 23-25 201
A convex polynomial that is not sos-convex
A multivariate polynomial is sos-convex if its Hessian
can be factored as with a possibly nonsquare
polynomial matrix . It is easy to see that sos-convexity is a sufficient
condition for convexity of . Moreover, the problem of deciding
sos-convexity of a polynomial can be cast as the feasibility of a semidefinite
program, which can be solved efficiently. Motivated by this computational
tractability, it has been recently speculated whether sos-convexity is also a
necessary condition for convexity of polynomials. In this paper, we give a
negative answer to this question by presenting an explicit example of a
trivariate homogeneous polynomial of degree eight that is convex but not
sos-convex. Interestingly, our example is found with software using sum of
squares programming techniques and the duality theory of semidefinite
optimization. As a byproduct of our numerical procedure, we obtain a simple
method for searching over a restricted family of nonnegative polynomials that
are not sums of squares.Comment: 15 page
The Synthesis of [{n-Bu2Sn(S2N2)}2] and its use in the preparation of Organometallic Iridium Sulfur Nitrogen Complexes
The addition of [n-Bu2SnCl2] to a solution of [S4N3][Cl] in liquid ammonia gave after extraction of the dry reaction mixture the new tin disulfur dinitrido compound [{n-Bu2Sn(S2N2)}(2)] (1). Reaction of [{n-Bu2Sn(S2N2)}(2)] (1) with the pentamethylcyclopentadienyl (Cp*) iridium derivatives [{IrCl(mu-Cl)(eta(5)-C5Me5)}(2)] or [(eta(5)-C5Me5)IrCl2(PPh3)] gave different products, which were dependent on the reactant ratios. A 1:1 reaction between 1 and [{IrCl(mu-Cl)(eta(5)-C5Me5)}(2)] gave only [(eta(5)-C5Me5)Ir(S2N2)] (2) in moderate yield; the same product in higher yield was obtained from a 2:1 reaction between 1 and [(eta(5)-C5Me5)IrCl2(PPh3)]. Reaction of 1 and [(eta(5)-C5Me5)(2)IrCl2(PPh3)] (1:1 molar ratio) in the presence of NH4[PF6] gave the unusual bimetallic species [(eta(5)-C5Me5)IrCl(PPh3)(S2N2)Ir(eta(5)-C5Me5)][PF6] (3). The X-ray crystal structures of 1, 2, and 3 are reported.PostprintPeer reviewe
Co-limitation towards lower latitudes shapes global forest diversity gradients
The latitudinal diversity gradient (LDG) is one of the most recognized global patterns of species richness exhibited across a wide range of taxa. Numerous hypotheses have been proposed in the past two centuries to explain LDG, but rigorous tests of the drivers of LDGs have been limited by a lack of high-quality global species richness data. Here we produce a high-resolution (0.025° × 0.025°) map of local tree species richness using a global forest inventory database with individual tree information and local biophysical characteristics from ~1.3 million sample plots. We then quantify drivers of local tree species richness patterns across latitudes. Generally, annual mean temperature was a dominant predictor of tree species richness, which is most consistent with the metabolic theory of biodiversity (MTB). However, MTB underestimated LDG in the tropics, where high species richness was also moderated by topographic, soil and anthropogenic factors operating at local scales. Given that local landscape variables operate synergistically with bioclimatic factors in shaping the global LDG pattern, we suggest that MTB be extended to account for co-limitation by subordinate drivers
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