Algebraic identity for the Schouten tensor and bi-Hamiltonian systems

A (0,3)-tensor Tijk is introduced in an invariant form. Algebraic identities are derived that connect the Schouten (2,1)-tensor and tensor Tijk with the Nijenhuis tensor . Applications to the bi-Hamiltonian dynamical systems are presented.     

A bi-Hamiltonian system on the Grassmannian

Considering the recent result that the Poisson–Nijenhuis geometry corresponds to the quantization of the symplectic groupoid integrating a Poisson manifold, we discuss the Poisson–Nijenhuis structure on the Grassmannian defined by the compatible Kirillov–Kostant–Souriau and Bruhat–Poisson structures. The eigenvalues of the Nijenhuis tensor are Gelfand–Tsetlin variables, which, as was proved, are also in involution with respect to the Bruhat–Poisson structure. Moreover, we show that the Stiefel bundle on the Grassmannian admits a bi-Hamiltonian structure.     

Non-degenerate para-complex structures in 6D with large symmetry groups

For an almost product structure J on a manifold M of dimension 6 with non-degenerate Nijenhuis tensor \(N_J\), we show that the automorphism group \(G=\mathrm{Aut}(M,J)\) has dimension at most 14. In the case of equality G is the exceptional Lie group \(G_2^*\). The next possible symmetry dimension is proved to be equal to 10, and G has Lie algebra \(\mathfrak {sp}(4,{\mathbb R})\). Both maximal and submaximal symmetric structures are globally homogeneous and strictly nearly para-Kähler. We also demonstrate that whenever the symmetry dimension is at least 9, then the automorphism algebra acts locally transitively.     

Feuilletages transversalement tangents

Summary A foliation on a differentiable manifold M is said to be transversally tangent if the jacobian matrices of the change of the transverse coordinates are in the tangent group. Such foliation exists if and only if there is an endomorphism J of its normal bundle such that J²=0 and such that the Nijenhuis tensor of J is the zero-tensor. In the special case of the tangent bundle of order 2, T²M, its total space has a natural (1, 1)-tensor F such that F³=0 and an integrable almost-tangent structure. We study several Lie algebras associated to these structures.     

Poisson-Nijenhuis structures and the Vinogradov bracket

We express the compatibility conditions that a Poisson bivector and a Nijenhuis tensor must fulfil in order to be a Poisson-Nijenhuis structure by means of a graded Lie bracket. This bracket is a generalization of Schouten and Frölicher-Nijenhuis graded Lie brackets defined on multivector fields and on vector valued differential forms respectively.Partially supported by Fundació Caixa Castelló.Partially supported by the Spanish DGICYT grant #P B91-0324.     

Null electromagnetic fields,total gravitational radiation and collineations in general relativity

Summary The null electromagnetic field, or (F, g, r, S)-structure, and the corresponding Nijenhuis tensor have been studied in an invariant index-free manner. It is seen that the null electromagnetic fields are characterized by the relation F³= 0, and the Nijenhuis tensor plays a very natural role in the study of null electromagnetic fields. The Lichnerowicz contions for the total gravitational radiation have been given in the present setting, and the condition F³=0 has been translated into a corresponding condition on the Ricci tensor. Further, different types of collineations, for (F, g, r, S)-structure, along the propagation and polarization vectors S and T, respectively, have been studied. It is also shown that implies . Finally, a covariant conservation law generator has been given for the present structure.The contents of this paper form a part of Ph. D. thesis of first author. Results of Sections 2 and 3 have been presented at the 8th Annual Conference of Indian Association for General Relativity and Gravitation, Bhavnagar (India), Dec. 31, 1977-Jan. 2, 1978, and that of Sections 4 and 5 at the Einstein Centenary Symposium, Physical Research Laboratory, Ahmadabad (India), Jan. 23-Feb. 3, 1979.Supported by a Senior Research Fellowship of Council of Scientific and Industrial Research (India) under Grant No. 7/112(536)/76-EMR-I.     

Integrable Submanifolds in Almost Complex Manifolds

Let (M,J) be a germ of an almost complex manifold of real dimension 2m and let n (n<m) be an integer. We study a necessary and sufficient condition for M to admit an integrable submanifold N of complex dimension n. If n=m−1, we find defining functions of N explicitly from the coefficients of the torsion tensor. For J obtained by small perturbation of the standard complex structure of ℂ ^m this condition is given as an overdetermined system of second order PDEs on the perturbation. The proof is based on the rank conditions of the Nijenhuis tensor and application of the Newlander-Nirenberg theorem. We give examples of almost complex structures on ℂ³: the ones with a single complex submanifold of dimension 2 and the ones with 1-parameter or 2-parameter families of complex submanifolds of dimension 2.     

Canonical connections on paracontact manifolds

The canonical paracontact connection is defined and it is shown that its torsion is the obstruction the paracontact manifold to be paraSasakian. A -homothetic transformation is determined as a special gauge transformation. The η-Einstein manifold are defined, it is proved that their scalar curvature is a constant, and it is shown that in the paraSasakian case these spaces can be obtained from Einstein paraSasakian manifolds with -homothetic transformations. It is shown that an almost paracontact structure admits a connection with totally skew-symmetric torsion if and only if the Nijenhuis tensor of the paracontact structure is skew-symmetric and the defining vector field is Killing.      

Classification locale simultanée de deux formes symplectiques compatibles

Consider a symplectic form ω and a closed 2-form ω₁ on a real or complex manifold. Suppose that the Nijenhuis torsion of the tensor fieldJ defined by ω₁(X,Y) = ω(JX,Y) vanishes. In this paper we give the complete local classification of the couple {ω, ω₁} on a dense open set, defined by some minor conditions of regularity. Around each point of this open set we can find coordinates on wich ω is written with constant coefficients and ω₁ with affine ones. Projet de recherche DGICYT PB91-0412     

Algebraic identities for the Nijenhuis tensors

The general algebraic identities are discovered for the Nijenhuis and Haantjes tensors on an arbitrary manifold Mn. For n=3, the special algebraic identities involving the symmetric bilinear form H(u,v) are derived.     

Hypersurfaces with isotropic para-Blaschke tensor

Let Mnbe an n-dimensional submanifold without umbilical points in the(n + 1)-dimensional unit sphere Sn+1.Four basic invariants of Mnunder the Moebius transformation group of Sn+1are a 1-form Φ called moebius form,a symmetric(0,2) tensor A called Blaschke tensor,a symmetric(0,2) tensor B called Moebius second fundamental form and a positive definite(0,2) tensor g called Moebius metric.A symmetric(0,2) tensor D = A + μB called para-Blaschke tensor,where μ is constant,is also an Moebius invariant.We call the para-Blaschke tensor is isotropic if there exists a function λ such that D = λg.One of the basic questions in Moebius geometry is to classify the hypersurfaces with isotropic para-Blaschke tensor.When λ is not constant,all hypersurfaces with isotropic para-Blaschke tensor are explicitly expressed in this paper.     

A special class of tensor categories initiated by inverse braid monoids

In this paper, we introduce the concept of a wide tensor category which is a special class of a tensor category initiated by the inverse braid monoids recently investigated by Easdown and Lavers [The Inverse Braid Monoid, Adv. in Math. 186 (2004) 438-455].The inverse braid monoidsIB_n is an inverse monoid which behaves as the symmetric inverse semigroup so that the braid group B_n can be regarded as the braids acting in the symmetric group. In this paper, the structure of inverse braid monoids is explained by using the language of categories. A partial algebra category, which is a subcategory of the representative category of a bialgebra, is given as an example of wide tensor categories. In addition, some elementary properties of wide tensor categories are given. The main result is to show that for every strongly wide tensor category C, a strict wide tensor category C^str can be constructed and is wide tensor equivalent to C with a wide tensor equivalence F.As a generalization of the universality property of the braid category B, we also illustrate a wide tensor category through the discussion on the universality of the inverse braid category IB (see Theorem 3.3, 3.6 and Proposition 3.7).     

An efficient algorithm for generating all k-subsets (1 ⩽ k ⩽ m ⩽ n) of the set [1, 2,…, n] in lexicographical order

We consider the problem of generating all k-subsets (1 ? k ? m ? n) of the set [1, 2, …, n] in lexicographical order. The running time per k-subset is shown to be constant. The experimental data suggest that our algorithm is about 25% faster than the algorithm proposed by A. Nijenhuis and H. S. Wilf (“Combinatorial Algorithms,” 2nd ed., Academic Press, New York, 1978).     

A Bochner formula for almost-quaternionic-Hermitian structures

An integral formula is derived, relating the six irreducible components of the intrinsic torsion of an Sp_nSp₁ structure on a compact 4n-dimensional manifold with the Riemann curvature tensor. Some consequences of the formula are studied.     

Eigenvalues and invariants of tensors

A tensor is represented by a supermatrix under a co-ordinate system. In this paper, we define E-eigenvalues and E-eigenvectors for tensors and supermatrices. By the resultant theory, we define the E-characteristic polynomial of a tensor. An E-eigenvalue of a tensor is a root of the E-characteristic polynomial. In the regular case, a complex number is an E-eigenvalue if and only if it is a root of the E-characteristic polynomial. We convert the E-characteristic polynomial of a tensor to a monic polynomial and show that the coefficients of that monic polynomial are invariants of that tensor, i.e., they are invariant under co-ordinate system changes. We call them principal invariants of that tensor. The maximum number of principal invariants of mth order n-dimensional tensors is a function of m and n. We denote it by d(m,n) and show that d(1,n)=1, d(2,n)=n, d(m,2)=m for m?3 and d(m,n)?mn−1+?+m for m,n?3. We also define the rank of a tensor. All real eigenvectors associated with nonzero E-eigenvalues are in a subspace with dimension equal to its rank.     

Maximal number of distinct H-eigenpairs for a two-dimensional real tensor

Based on the generalized characteristic polynomial introduced by J. Canny in Generalized characteristic polynomials [J. Symbolic Comput., 1990, 9(3): 241–250], it is immediate that for any m-order n-dimensional real tensor, the number of distinct H-eigenvalues is less than or equal to n(m?1) ^n?1. However, there is no known bounds on the maximal number of distinct Heigenvectors in general. We prove that for any m ? 2, an m-order 2-dimensional tensor A exists such that A has 2(m ? 1) distinct H-eigenpairs. We give examples of 4-order 2-dimensional tensors with six distinct H-eigenvalues as well as six distinct H-eigenvectors. We demonstrate the structure of eigenpairs for a higher order tensor is far more complicated than that of a matrix. Furthermore, we introduce a new class of weakly symmetric tensors, called p-symmetric tensors, and show under certain conditions, p-symmetry will effectively reduce the maximal number of distinct H-eigenvectors for a given two-dimensional tensor. Lastly, we provide a complete classification of the H-eigenvectors of a given 4-order 2-dimensional nonnegative p-symmetric tensor. Additionally, we give sufficient conditions which prevent a given 4-order 2-dimensional nonnegative irreducible weakly symmetric tensor from possessing six pairwise distinct H-eigenvectors.     

Nijenhuis forms on Lie-infinity algebras associated to Lie algebroids

Introducing Nijenhuis forms on L_∞-algebras gives a general frame to understand deformations of the latter. We give here a Nijenhuis interpretation of a deformation of an arbitrary Lie algebroid into an L_∞-algebra. Then we show that Nijenhuis forms on L_∞-algebras also give a short and e?cient manner to understand Poisson-Nijenhuis structures and, more generally, the so-called exact Poisson quasi-Nijenhuis structures with background.     

Compatible Structures on Lie Algebroids and Monge-Ampère Operators

We study pairs of structures, such as the Poisson-Nijenhuis structures, on the tangent bundle of a manifold or, more generally, on a Lie algebroid or a Courant algebroid. These composite structures are defined by two of the following, a closed 2-form, a Poisson bivector or a Nijenhuis tensor, with suitable compatibility assumptions. We establish the relationships between PN-, P Ω- and Ω N-structures. We then show that the non-degenerate Monge-Ampère structures on 2-dimensional manifolds satisfying an integrability condition provide numerous examples of such structures, while in the case of 3-dimensional manifolds, such Monge-Ampère operators give rise to generalized complex structures or generalized product structures on the cotangent bundle of the manifold.     

Homogeneous almost complex structures in dimension 6 with semi-simple isotropy

We classify invariant almost complex structures on homogeneous manifolds of dimension 6 with semi-simple isotropy. Those with non-degenerate Nijenhuis tensor have the automorphism group of dimension either 14 or 9. An invariant almost complex structure with semi-simple isotropy is necessarily either of specified 6 homogeneous types or a left-invariant structure on a Lie group. For integrable invariant almost complex structures we classify all compatible invariant Hermitian structures on these homogeneous manifolds, indicate their integrability properties (Kähler, SNK, SKT) and mark the other interesting geometric properties (including the Gray-Hervella type).