## Search

Now showing items 1-10 of 48

#### Rigidity of 2-step carnot groups

(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-19)

In the present paper we study the rigidity of 2-step Carnot groups, or equivalently, of graded 2-step nilpotent Lie algebras. We prove the alternative that depending on bi-dimensions of the algebra, the Lie algebra structure makes it either always of inﬁnite type or generically rigid, and we specify the bi-dimensions for each of the choices. Explicit criteria for rigidity of pseudo Hand <i>J</i>-type ...

#### Dynamics and entropy in the Zhang model of Self-Organized Criticality

(Working paper; Arbeidsnotat, 2005-09-12)

We give a detailed study of dynamical properties of the Zhang model,
including evaluation of topological entropy and estimates for the Lyapunov
exponents and the dimension of the attractor. In the thermodynamic
limit the entropy goes to zero and the Lyapunov spectrum collapses.1

#### Entropy via multiplicity

(Working paper; Arbeidsnotat, 2005-09-30)

The topological entropy of piecewise affine maps is studied. It is shown
that singularities may contribute to the entropy only if there is angular expansion
and we bound the entropy via the expansion rates of the map. As
a corollary we deduce that non-expanding conformal piecewise affine maps
have zero topological entropy. We estimate the entropy of piecewise affine
skew-products. Examples of ...

#### A piece-wise affine contracting map with positive entropy

(Working paper; Arbeidsnotat, 2005-04-10)

We construct the simplest chaotic system with a two-point attractor.

#### SDiff(2) and uniqueness of the Plebanski equation

(Journal article; Tidsskriftartikkel; Peer reviewed, 2012)

The group of area preserving diffeomorphisms showed importance in the problems of self-dual gravity and integrability theory. We discuss how representations of this infinite-dimensional Lie group can arise in mathematical physics from pure local considerations. Then using Lie algebra extensions and cohomology we derive the second Plebański equation and its geometry. We do not use Kähler or other ...

#### The gap phenomenon in parabolic geometries

(Journal article; Tidsskriftartikkel; Peer reviewed, 2014-09-14)

The infinitesimal symmetry algebra of any Cartan geometry has maximum dimension realized by the flat model, but often this dimension drops significantly when considering non-flat geometries, so a gap phenomenon arises. For general (regular, normal) parabolic geometries of type (G,P), we use Tanaka theory to derive a universal upper bound on the submaximal symmetry dimension. We use Kostant’s version ...

#### Conformal differential invariants

(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-27)

We compute the Hilbert polynomial and the Poincar´e
function counting the number of fixed jet-order differential invariants
of conformal metric structures modulo local diffeomorphisms,
and we describe the field of rational differential invariants separating
generic orbits of the diffeomorphism pseudogroup action. This
resolves the local recognition problem for conformal structures.

#### On a class of integrable systems of Monge-Ampère type

(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-06-08)

We investigate a class of multi-dimensional two-component systems of Monge-Ampère type that can be viewed as generalisations of heavenly type equations appearing in a self-dual Ricci-flat geometry. Based on the Jordan-Kronecker theory of the skew-symmetric matrix pencils, a classification of normal forms of such systems is obtained. All two-component systems of Monge-Ampère type turn out to be ...

#### Differential Invariants of Linear Symplectic Actions

(Journal article; Tidsskriftartikkel; Peer reviewed, 2020-12-07)

We consider the equivalence problem for symplectic and conformal symplectic group actions on submanifolds and functions of symplectic and contact linear spaces. This is solved by computing differential invariants via the Lie-Tresse theorem.

#### On integrability of certain rank 2 sub-Riemannian structures

(Journal article; Tidsskriftartikkel; Peer reviewed, 2017-10-01)

We discuss rank 2 sub-Riemannian structures on low-dimensional manifolds and prove that some of these structures in dimensions 6, 7 and 8 have a maximal amount of symmetry but no integrals polynomial in momenta of low degrees, except for those coming from the Killing vector fields and the Hamiltonian, thus indicating nonintegrability of the corresponding geodesic flows.