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- ItemYet another algorithm for the symmetric eigenvalue problem(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2016) Aurentz, Jared L.; Mach, Thomas; Vandebril, Raf; Watkins, David S.In this paper we present a new algorithm for solving the symmetric matrix eigenvalue problem that works by first using a Cayley transformation to convert the symmetric matrix into a unitary one and then uses Gragg’s implicitly shifted unitary QR algorithm to solve the resulting unitary eigenvalue problem. We prove that under reasonable assumptions on the symmetric matrix this algorithm is backward stable and also demonstrate that this algorithm is comparable with other well known implementations in terms of both speed and accuracy.
- ItemCrystal energy functions via the charge in types A and C(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Lenart, Cristian; Schilling, AnneThe Ram-Yip formula for Macdonald polynomials (at t=0) provides a statistic which we call charge. In types A and C it can be defined on tensor products of Kashiwara-Nakashima single column crystals. In this paper we prove that the charge is equal to the (negative of the) energy function on affine crystals. The algorithm for computing charge is much simpler and can be more efficiently computed than the recursive definition of energy in terms of the combinatorial R-matrix.
- ItemAbundance of 3-planes on real projective hypersurfaces(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2014) Finashin, S.; Kharlamov, V.We show that a generic real projective n-dimensional hypersurface of odd degree d , such that 4(n - 2) = (d + 3 3), contains "many" real 3-planes, namely, in the logarithmic scale their number has the same rate of growth, d3 log d, as the number of complex 3-planes. This estimate is based on the interpretation of a suitable signed count of the 3-planes as the Euler number of an appropriate bundle.
- ItemThe algebra of differential operators for a Gegenbauer weight matrix(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2015) Zurrián, Ignacio NahuelIn this work we study in detail the algebra of differential operators D(W) associated with a Gegenbauer matrix weight. We prove that two second order operators generate the algebra, indeed D(W) is isomorphic to the free algebra generated by two elements subject to certain relations. Also, the center is isomorphic to the affine algebra of a singular rational curve. The algebra D(W) is a finitely-generated torsion-free module over its center, but it is not at and therefore neither projective. After [Tir11], this is the second detailed study of an algebra D(W) and the first one coming from spherical functions and group representation theory.
- ItemThe initial and terminal cluster sets of an analytic curve(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2016) Gauthier, PaulFor an analytic curve γ:(a,b)→C, the set of values approaches by γ(t), as t↘a and as t↗b can be any two continuua of C∪{∞}.
- ItemObservability of systems with delay convoluted observation(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2014) Ivanov, Anatoli F.; Verriest, Erik I.This paper analyzes finite dimensional linear time-invariant systems with observation of a delay, where that delay satisfies a particular implicit relation with the state variables, rendering the entire problem nonlinear. The objective is to retrieve the state variables from the measured delay. The first contribution involves the direct inversion of the delay, the second is the design of a finite dimensional state observer, and the third involves the derivation of certain properties of the delay - state relation. Realistic examples treat vehicles with ultrasonic position sensors
- ItemThe index of singular zeros of harmonic mappings of anti-analytic degree one(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2017) Luce, Robert; Sète, OlivierWe study harmonic mappings of the form f(z)=h(z)−z¯¯¯, where h is an analytic function. In particular we are interested in the index (a generalized multiplicity) of the zeros of such functions. Outside the critical set of f, where the Jacobian of f is non-vanishing, it is known that this index has similar properties as the classical multiplicity of zeros of analytic functions. Little is known about the index of zeros on the critical set, where the Jacobian vanishes; such zeros are called singular zeros. Our main result is a characterization of the index of singular zeros, which enables one to determine the index directly from the power series of h.
- ItemInvariants of closed braids via counting surfaces(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2012) Brandenbursky, MichaelA Gauss diagram is a simple, combinatorial way to present a link. It is known that any Vassiliev invariant may be obtained from a Gauss diagram formula that involves counting subdiagrams of certain combinatorial types. In this paper we present simple formulas for an infinite family of invariants in terms of counting surfaces of a certain genus and number of boundary components in a Gauss diagram associated with a closed braid. We then identify the resulting invariants with partial derivatives of the HOMFLY-PT polynomial.
- ItemA generalization of the discrete version of Minkowski’s Fundamental Theorem(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2014) González Merino, Bernardo; Henze, MatthiasOne of the most fruitful results from Minkowski’s geometric viewpoint on number theory is his so called 1st Fundamental Theorem. It provides an optimal upper bound for the volume of an o-symmetric convex body whose only interior lattice point is the origin. Minkowski also obtained a discrete analog by proving optimal upper bounds on the number of lattice points in the boundary of such convex bodies. Whereas the volume inequality has been generalized to any number of interior lattice points already by van der Corput in the 1930s, a corresponding result for the discrete case remained to be proven. Our main contribution is a corresponding optimal relation between the number of boundary and interior lattice points of an o-symmetric convex body. The proof relies on a congruence argument and a difference set estimate from additive combinatorics.
- ItemAn identification theorem for PSU6(2) and its automorphism groups(Oberwolfach : Mathematisches Forschungsinstitut Oberwolfach, 2011) Parker, Chris; Stroth, GernotWe identify the groups PSU6(2), PSU6(2):2, PSU6(2):3 and Aut(PSU6(2)) from the structure of the centralizer of an element of order 3.