On the Energy of Distributions, with Application to the Quaternionic Hopf Fibrations

 The energy of an oriented q-distribution ? in a compact oriented manifold M is defined to be the energy of the section of the Grassmannian manifold of oriented q-planes in M induced by ?. In the Grassmannian, the Sasaki metric is considered. We show here a condition for a distribution to be a critical point of the energy functional. In the spheres, we see that Hopf fibrations are critical points. Later, we prove the instability for these fibrations. (Received 30 December 2000; in revised form 11 April 2001)     

Some Geometric Characterizations of the Hopf Fibrations of the Three-Sphere

The Fréchet manifold of all embeddings (up to orientation preserving reparametrizations) of the circle in S ³ has a canonical weak Riemannian metric. We use the characterization obtained by H. Gluck and F. Warner of the oriented great circle fibrations of S ³ to prove that among all such fibrations π:S ³→B, the manifold B consisting of the oriented fibers is totally geodesic in , or has minimum volume or diameter with the induced metric, exactly when π is a Hopf fibration. Partially supported by foncyt, Antorchas, ciem (conicet) and secyt (unc).     

Some Geometric Characterizations of the Hopf Fibrations of the Three-Sphere

The Fréchet manifold E/ _~ {\cal E}/_{\!\sim} of all embeddings (up to orientation preserving reparametrizations) of the circle in S ³ has a canonical weak Riemannian metric. We use the characterization obtained by H. Gluck and F. Warner of the oriented great circle fibrations of S ³ to prove that among all such fibrations π:S ³→B, the manifold B consisting of the oriented fibers is totally geodesic in E/ _~ {\cal E}/_{\sim } , or has minimum volume or diameter with the induced metric, exactly when π is a Hopf fibration.     

The Subelliptic Heat Kernels of the Quaternionic Hopf Fibration

The main goal of this work is to study the sub-Laplacian of the unit sphere which is obtained by lifting with respect to the Hopf fibration the Laplacian of the quaternionic projective space. We obtain in particular explicit formulas for its heat kernel and deduce an expression for the Green function of the conformal sub-Laplacian and small-time asymptotics. As a byproduct of our study we also obtain several results related to the sub-Laplacian of a projected Hopf fibration.     

On Milnor Fibrations of Arrangements

We use covering space theory and homology with local coefficientsto study the Milnor fiber of a homogeneous polynomial. Thesetechniques are applied in the context of hyperplane arrangements,yielding an explicit algorithm for computing the Betti numbersof the Milnor fiber of an arbitrary real central arrangementin C³, as well as the dimensions of the eigenspaces of the algebraicmonodromy. We also obtain combinatorial formulas for these invariantsof the Milnor fiber of a generic arrangement of arbitrary dimensionusing these methods.     

On the Topology of Fibrations with Section and Free Loop Spaces

We relate the brace products of a fibration with section tothe differentials in its Serre spectral sequence. In the particularcase of free loop fibrations, we establish a link between thesedifferentials and Browder operations in the fiber. Applicationsand several calculations (for the particular cases of spheresand wedges of spheres) are given. 2000 Mathematical SubjectClassification: 55P35, 55Q15, 55R05, 55R20.     

Bayesian Copulae Distributions, with Application to Operational Risk Management

The aim of this paper is to introduce a new methodology for operational risk management, based on Bayesian copulae. One of the main problems related to operational risk management is understanding the complex dependence structure of the associated variables. In order to model this structure in a flexible way, we construct a method based on copulae. This allows us to split the joint multivariate probability distribution of a random vector of losses into individual components characterized by univariate marginals. Thus, copula functions embody all the information about the correlation between variables and provide a useful technique for modelling the dependency of a high number of marginals. Another important problem in operational risk modelling is the lack of loss data. This suggests the use of Bayesian models, computed via simulation methods and, in particular, Markov chain Monte Carlo. We propose a new methodology for modelling operational risk and for estimating the required capital. This methodology combines the use of copulae and Bayesian models.      

On the Multiplicity of Zeroes of Polynomials with Quaternionic Coefficients

Regular polynomials with quaternionic coefficients admit only isolated zeroes and spherical zeroes. In this paper we prove a factorization theorem for such polynomials. Specifically, we show that every regular polynomial can be written as a product of degree one binomials and special second degree polynomials with real coefficients. The degree one binomials are determined (but not uniquely) by the knowledge of the isolated zeroes of the original polynomial, while the second degree factors are uniquely determined by the spherical zeroes. We also show that the number of zeroes of a polynomial, counted with their multiplicity as defined in this paper, equals the degree of the polynomial. While some of these results are known in the general setting of an arbitrary division ring, our proofs are based on the theory of regular functions of a quaternionic variable, and as such they are elementary in nature and offer explicit constructions in the quaternionic setting. Partially supported by G.N.S.A.G.A.of the I.N.D.A.M. and by M.I.U.R.. Lecture held by G. Gentili in the Seminario Matematico e Fisico on February 12, 2007. Received: August 2008     

On the Derivatives of Quaternionic Functions Along Two-Dimensional Planes

In a previous paper we introduced the concept of a two-dimensional directional derivative of a quaternionic function along a two-dimensional plane. In this paper we provide a deeper analysis of its properties, as well as of its relations with hyperholomorphic functions, with holomorphic maps of two complex variables and with Cullen-regular functions. Received: October, 2007. Accepted: February, 2008.     

On Normal Hopf Subalgebras of Semisimple Hopf Algebras

A criterion for subcoalgebras to be invariant under the adjoint action is given generalizing Masuoka’s criterion for normal Hopf subalgebras. At the level of characters, the image of the induction functor from a normal Hopf subalgebra is isomorphic to the image of the restriction functor.     

On the harmonic Hopf construction

The harmonic Hopf construction is an equivariant ansatz for harmonic maps between Euclidean spheres. We prove existence of solutions in the case that has been open. Moreover, we show that the harmonic Hopf construction on every bi-eigenmap with at least one large eigenvalue has a countable family of solutions (if it has one).

     

On the transition from supercritical to subcritical Hopf bifurcations

By using the method of averaging, we give a complete Hopf bifurcation diagram when the differential equation has more than one parameter. The method is applied to a system of parabolic equations with nonlinear coupling on the boundary. This set of equations is a mathematical model which describes the normalized concentrations of substances in a solution produced by two enzymes.