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- ItemExtremal configurations of polygonal linkages(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Khimshiashvili, G.; Panina, G.; Siersma, D.; Zhukova, A.[no abstract available]
- ItemInfeasibility certificates for linear matrix inequalities(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Klep, Igor; Schweighofer, MarkusFarkas' lemma is a fundamental result from linear programming providing linear certi cates for infeasibility of systems of linear inequalities. In semidefinite programming, such linear certificates only exist for strongly infeasible linear matrix inequalities. We provide nonlinear algebraic certificates for all infeasible linear matrix inequalities in the spirit of real algebraic geometry. More precisely, we show that a linear matrix inequality L(x)⪰0 is infeasible if and only if −1 lies in the quadratic module associated to L. We prove exponential degree bounds for the corresponding algebraic certificate. In order to get a polynomial size certi cate, we use a more involved algebraic certificate motivated by the real radical and Prestel's theory of semiorderings. Completely different methods, namely complete positivity from operator algebras, are employed to consider linear matrix inequality domination.
- ItemA series of algebras generalizing the octonions and Hurwitz-Radon identity(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2010) Morier-Genoud, Sophie; Ovsienko, ValentinWe study non-associative twisted group algebras over (Z2)n with cubic twisting functions. We construct a series of algebras that extend the classical algebra of octonions in the same way as the Clifford algebras extend the algebra of quaternions. We study their properties, give several equivalent definitions and prove their uniqueness within some natural assumptions. We then prove a simplicity criterion. We present two applications of the constructed algebras and the developed technique. The first application is a simple explicit formula for the following famous square identity: (a21+...+a2N)(b21+...+b2ρ(N))=c21+...+c2N, where ck are bilinear functions of the ai and bj and where ρ(N) is the Hurwitz-Radon function. The second application is the relation to Moufang loops and, in particular, to the code loops. To illustrate this relation, we provide an explicit coordinate formula for the factor set of the Parker loop.
- ItemA construction of hyperbolic coxeter groups(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2010) Osajda, DamianWe give a simple construction of Gromov hyperbolic Coxeter groups of arbitrarily large virtual cohomological dimension. Our construction provides new examples of such groups. Using this one can construct e.g. new groups having some interesting asphericity properties.
- ItemA new counting function for the zeros of holomorphic curves(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2009) Anderson, J.M.; Hinkkanen, AimoLet f1, . . . , fp be entire functions that do not all vanish at any point, so that (f1, . . . , fp) is a holomorphic curve in CPp−1. We introduce a new and more careful notion of counting the order of the zero of a linear combination of the functions f1, . . . , fp at any point where such a linear combination vanishes, and, if all the f1, . . . , fp are polynomials, also at infinity. This enables us to formulate an inequality, which sometimes holds as an identity, that sharpens the classical results of Cartan and others.
- ItemLocally conformally Kähler manifolds admitting a holomorphic conformal flow(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2010) Ornea, Liviu; Verbitsky, MishaAbstract A manifold M is locally conformally Kähler (LCK) if it admits a Kähler covering ˜M with monodromy acting by holomorphic homotheties. Let M be an LCK manifold admitting a holomorphic conformal flow of diffeomorphisms, lifted to a non-isometric homothetic flow on ˜M . We show that M admits an automorphic potential, and the monodromy group of its conformal weight bundle is Z.
- ItemThe contact polytope of the Leech lattice(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2009) Dutour Sikiri´c, Mathieu; Schürmann, Achill; Vallentin, FrankThe contact polytope of a lattice is the convex hull of its shortest vectors. In this paper we classify the facets of the contact polytope of the Leech lattice up to symmetry. There are 1, 197, 362, 269, 604, 214, 277, 200 many facets in 232 orbits.
- ItemOn test sets for nonlinear integer maximization(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2007) Lee, Jon; Onn, Shmuel; Weismantel, RobertA finite test set for an integer maximization problem enables us to verify whether a feasible point attains the global maximum. We estabish in the paper several general results that apply to integer maximization problems wthe monlinear objective functions.
- ItemLegendrian knots in Lens spaces(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Onaran, Sinem CelikIn this note, we first classify all topological torus knots lying on the Heegaard torus in Lens spaces, and then we classify Legendrian representatives of torus knots. We show that all Legendrian torus knots in universally tight contact structures on Lens spaces are determined up to contactomorphism by their knot type, rational Thurston-Bennequin invariant and rational rotation number.
- ItemNew representations of matroids and generalizations(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Izhakian, Zur; Rhodes, JohnWe extend the notion of matroid representations by matrices over fields by considering new representations of matroids by matrices over finite semirings, more precisely over the boolean and the superboolean semirings. This idea of representations is naturally generalized to include hereditary collections (also known as abstract simplicial complexes). We show that a matroid that can be directly decomposed as matroids, each of which is representable over a field, has a boolean representation, and more generally that any arbitrary hereditary collection is superboolean-representable.