On the composition of convex envelopes for quadrilinear terms

International audienceWithin the framework of the spatial Branch-and-Bound algorithm for solving Mixed-Integer Nonlinear Programs, different convex relaxations can be obtained for multilinear terms by applying associativity in different ways. The two groupings ((x1x2)x3)x4 and (x1x2x3)x4 of a quadrilinear term, for example, give rise to two different convex relaxations. In [6] we prove that having fewer groupings of longer terms yields tighter convex relaxations. In this paper we give an alternative proof of the same fact and perform a computational study to assess the impact of the tightened convex relaxation in a spatial Branch-and-Bound setting

Compact relaxations for polynomial programming problems

Reduced RLT constraints are a special class of Reformulation- Linearization Technique (RLT) constraints. They apply to nonconvex (both continuous and mixed-integer) quadratic programming problems subject to systems of linear equality constraints. We present an extension to the general case of polynomial programming problems and discuss the derived convex relaxation. We then show how to perform rRLT constraint generation so as to reduce the number of inequality constraints in the relaxation, thereby making it more compact and faster to solve. We present some computational results validating our approach

Finding and testing network communities by lumped Markov chains

Identifying communities (or clusters), namely groups of nodes with comparatively strong internal connectivity, is a fundamental task for deeply understanding the structure and function of a network. Yet, there is a lack of formal criteria for defining communities and for testing their significance. We propose a sharp definition which is based on a significance threshold. By means of a lumped Markov chain model of a random walker, a quality measure called "persistence probability" is associated to a cluster. Then the cluster is defined as an "$\alpha$-community" if such a probability is not smaller than $\alpha$. Consistently, a partition composed of $\alpha$-communities is an "$\alpha$-partition". These definitions turn out to be very effective for finding and testing communities. If a set of candidate partitions is available, setting the desired $\alpha$-level allows one to immediately select the $\alpha$-partition with the finest decomposition. Simultaneously, the persistence probabilities quantify the significance of each single community. Given its ability in individually assessing the quality of each cluster, this approach can also disclose single well-defined communities even in networks which overall do not possess a definite clusterized structure

Flying safely by bilevel programming

International audiencePreventing aircraft from getting too close to each other is an essential element of safety of the air transportation industry, which becomes ever more important as the air traffic increases. The problem consists in enforcing a minimum distance threshold between flying aircraft, which naturally results in a bilevel formulation with a lower-level subproblem for each pair of aircraft. We propose two single-level reformulations, present a cut generation algorithm which directly solves the bilevel formulation and discuss comparative computational results

6-bromo-5-hydroxy-3-indolecarboxyaldehyde From the Caribbean Sponge Oceanapia-bartschi

The organic extract from Oceanapia bartschi has been shown to contain the antibiotic diterpene Ambliol A (1) and three indole derivatives, 3-bromoindole (2), 6-bromo-3-indolecarboxyaldehyde (3), and 6-bromo-5-hydroxy-3-indolecarboxyaldehyde (4). The structure of the novel compound 4 has been established from its spectroscopic (H-1 and C-13 NMR, UV, IR, and MS) data