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Joel Merker

Publications and source records attributed to Joel Merker.

At least 55 records · Page 3Linked to original sources
arXiv2011

A Groebner-bases algorithm for the computation of the cohomology of Lie (super) algebras

We present an effective algorithm for computing the standard cohomology spaces of finitely generated Lie (super) algebras over a commutative field K of characteristic zero. In order to reach explicit representatives of some generators of the quotient space Z^k/B^k of cocycles Z^k modulo coboundaries B^k, we apply Groebner bases techniques (in the appropriate linear setting) and take advantage of their strength. Moreover, when the considered Lie (super) algebras enjoy a grading -- a case which often happens both in representation theory and in differential geometry --, all cohomology spaces Z^k/B^k naturally split up as direct sums of smaller subspaces, and this enables us, for higher dimensional Lie (super) algebras, to improve the computer speed of calculations. Lastly, we implement our algorithm in the Maple software and evaluate its performances via some examples, most of which have several applications in the theory of Cartan-Tanaka connections.

math.AC↗
arXiv2011

Effective Cartan-Tanaka connections on C6-smooth strongly pseudoconvex hypersurfaces M3 of C2

In a recent expository article (Notices of the AMS, 58 (2011), no. 1, 20-27), Ezhov, McLaughlin and Schmalz showed how to perform in an effective way Tanaka's prolongation procedure valid generally for filtered structures of constant type when the distribution is equipped with an integrable complex structure, so as to derive the principal curvature invariants and (co)frame(s) associated to strongly pseudoconvex real hypersurfaces M3 in C2, an approach which is alternative and complementary to the `hyperspherical' connection of Elie Cartan, and to the so-called calculi of Fefferman, of Chern-Moser, of Webster. By choosing an initial frame for TM which is explicit in terms of a local graphing function v = f(x, y, u) for M, we provide a Cartan-Tanaka connection all elements of which are completely explicit in terms of f(x, y, u), assuming only C6-smoothness of M. The Gaussian requirement for systematic computational effectiveness then shows - a bit unexpectedly - that the two main curvatures are rational differential expressions in the sixth-order jet of f(x, y, u), the lengths of which are about 1000 pages long on a computer - just for the simplest instance of local embedded CR geometry. Large parts of the memoir aim at formulating general statements that will be useful for further constructions of Cartan-Tanaka connections related to the equivalence problem for (local) embedded CR manifolds whose CR-automorphism group is not semi-simple, cf. e.g. some model lists by Beloshapka.

math.DG↗
arXiv2010

Applications of computational invariant theory to Kobayashi hyperbolicity and to Green-Griffiths algebraic degeneracy

A major unsolved problem (according to Demailly 1997) towards the Kobayashi hyperbolicity conjecture in optimal degree is to understand jet differentials of germs of holomorphic discs that are invariant under any reparametrization of the source. The underlying group action is not reductive, but we provide a complete algorithm to generate all invariants, in arbitrary dimension n and for jets of arbitrary order k. Two main new situations are studied in great details. For jets of order 4 in dimension 4, we establish that the algebra of Demailly-Semple invariants is generated by 2835 polynomials, while the algebra of bi-invariants is generated by 16 mutually independent polynomials sharing 41 groebnerized syzygies. Nonconstant entire holomorphic curves valued in an algebraic 3-fold (resp. 4-fold) X^3 in P^4 (C) (resp. X^4 in P^5(C)) of degree d satisfy global differential equations as soon as d > 71 (resp. d > 258). A useful asymptotic formula for the Euler-Poincare characteristic of Schur bundles in terms of Giambelli's determinants is derived. For jets of order 5 in dimension 2, we establish that the algebra of Demailly-Semple invariants is generated by 56 polynomials, while the algebra of bi-invariants is generated by 17 mutually independent polynomials sharing 105 groebnerized syzygies.

math.AG↗
arXiv2010

Effective algebraic degeneracy

We prove that any nonconstant entire holomorphic curve from the complex line C into a projective algebraic hypersurface X = X^n in P^{n+1}(C) of arbitrary dimension n (at least 2) must be algebraically degenerate provided X is generic if its degree d = deg(X) satisfies the effective lower bound: d larger than or equal to n^{{(n+1)}^{n+5}}.

math.AG↗
arXiv2010

Nonrigid spherical real analytic hypersurfaces in C^2

A Levi nondegenerate real analytic hypersurface M of C^2 represented in local coordinates (z, w) in C^2 by a complex defining equation of the form w = Theta (z, \bar z, \bar w) which satisfies an appropriate reality condition, is spherical if and only if its complex graphing function Theta satisfies an explicitly written sixth-order polynomial complex partial differential equation. In the rigid case (known before), this system simplifies considerably, but in the general nonrigid case, its combinatorial complexity shows well why the two fundamental curvature tensors constructed by Elie Cartan in 1932 in his classification of hypersurfaces have, since then, never been reached in parametric representation.

math.CV↗
arXiv2009

Vanishing Hachtroudi curvature and local equivalence to the Heisenberg sphere

To any completely integrable second-order system of real or complex partial differential equations in n > 1 independent variables and in one dependent variable, Mohsen Hachtroudi associated in 1937 a normal projective (Cartan) connection, and he computed its curvature. By means of a natural transfer of jet polynomials to the associated submanifold of solutions, what the vanishing of the Hachtroudi curvature gives can be precisely translated in order to characterize when both families of Segre varieties and of conjugate Segre varieties associated to a Levi nondegenerate real analytic hypersurface M in C^{n+1} can be straightened to be affine complex (conjugate) hyperplanes. In continuation to a previous paper devoted to the quite distinct C^2-case, this then characterizes in an effective way those hypersurfaces of C^{n+1} in higher complex dimension n+1 > 2 that are locally biholomorphic to a piece of the (2n+1)-dimensional Heisenberg sphere, without any special assumption on their defining equations.

math.CV↗
arXiv2008

Low pole order frames on vertical jets of the universal hypersurface

Of the two techniques introduced by Bloch, Green-Griffiths and developed by Siu, Demailly to establish Kobayashi hyperbolicity of generic high degree complex algebraic hypersurfaces X in P^(n+1), the second one, initiated by Clemens, Ein, Voisin and developed by Siu, Paun, Rousseau consists in constructing meromorphic frames on the space of the so-called vertical k-jets J_vert^k (X_univ) of the universal hypersurface X_univ parametrizing all X in P^(n+1) of degree d. In 2004, Siu announced that there exists a constant c_n such that the twisting of the tangent bundle to J_vert^n (X_univ) by O (c_n) is globally generated (frame property). The present article provides c_n = (n^2 + 5n) / 2, recovering c_2 = 7 (Paun), c_3 = 12 (Rousseau). Applications to effective degree estimates for algebraic degeneracy or hyperbolicity are expected, especially in dimension n = 4, granted that the Demailly-Semple algebra of jet polynomials invariant under reparametrization and under a certain unipotent action is, for n = k = 4, generated by 16 fundamental bi-invariants enjoying 41 groebnerized syzygies.

math.AG↗
arXiv2008

On Wittgenstein's philosophy of mathematics

Mathematics cannot anymore be assimilated to a linguistic game, where formal proofs are strongly differentiated with conjectural thinking, without building any category of knowledge to understand the passage (Wittgenstein's gist). Nowadays, philosophy has to face with the growing, exponential ramified tree of speculative mathematical thinking. Our main (problematical) theses are: 1. In mathematics, there is no empirical automatism, and no separate, physical-like motricity. 2: The irreversible-synthetical must force to complexify the exegetical game of philosophy; numerical experiments in algebra and in number theory are a kind of letting blow up all possible problems; 4. The nature of mathematical questioning still remains in question.

math.HO↗
arXiv2007

The Hartogs extension theorem on (n-1)-complete complex spaces

Employing Morse theory for the global control of monodromy and the method of analytic discs for local extension, we establish a version of the global Hartogs extension theorem in a singular setting: for every domain D of an (n-1)-complete normal complex space X of pure dimension n >= 2 and for every compact set K in D such that D - K is connected, holomorphic or meromorphic functions in D - K extend holomorphically or meromorphically to D. Normality is an unvavoidable assumption for holomorphic extension, but we show that meromorphic extension holds on a reduced globally irreducible (not necessarily normal) X of pure dimension n >=2 provided that the regular part of D - K is connected.

math.CV↗
arXiv2007

I: Lie symmetries of partial differential equations and CR geometry

A general theory of rigid completely integrable analytic partial differential equations is endeavoured. The tube over the light cone in C^3 is shown to be the unique model (up to biholomorphisms) having CR automorphism group of maximal dimension equal to 10. Explicit formulas for the Lie prolongation of vector fields to jets spaces are derived.

math.CV↗
arXiv2006

Explicit differential characterization of the Newtonian free particle system in m > 1 dependent variables

In 1883, as an early result, Sophus Lie established an explicit necessary and sufficient condition for an analytic second order ordinary differential equation y_xx = F(x,y,y_x) to be equivalent, through a point transformation (x,y) --> (X(x,y), Y(x,y)), to the Newtonian free particle equation Y_XX = 0. This result, preliminary to the deep group-theoretic classification of second order analytic ordinary differential equations, was parachieved later in 1896 by Arthur Tresse, a French student of S. Lie. In the present paper, following closely the original strategy of proof of S. Lie, which we firstly expose and restitute in length, we generalize this explicit characterization to the case of several second order ordinary differential equations. Let K=R or C, or more generally any field of characteristic zero equipped with a valuation, so that K-analytic functions make sense. Let x in K, let m > 1, let y := (y^1, ..., y^m) in K^m and let y_xx^j = F^j(x,y,y_x^l), j = 1,...,m be a collection of m analytic second order ordinary differential equations, in general nonlinear. We provide an explicit necessary and sufficient condition in order that this system is equivalent, under a point transformation (x, y^1, ..., y^m) --> (X(x,y), Y^1(x,y),..., Y^m(x, y)), to the Newtonian free particle system Y_XX^1 = ... = Y_XX^m = 0. Strikingly, the (complicated) differential system that we obtain is of first order in the case m > 1, whereas it is of second order in S. Lie's original case m = 1.

math.DG↗
arXiv2005

Explicit differential characterization of PDE systems pointwise equivalent to Y_{X^{j_1}X^{j_2}}=0, 1\leq j_1,j_2\leq n\geq 2

In this paper, a direct continuation of math.DG/0411165, we generalize S. Lie's linearization criterion of an ordinary second order differential equation to the case of several independent variables (x^1, x^2 ..., x^n), n >1, and a single dependent variable y. Strikingly, as in math.DG/0411165, the (complicated) characterizing differential system is of first order. By means of computer programming, this phenomenon was discovered in the case n=2 by S. Neut and M. Petitot (www.lifl.fr/~neut/recherche/these.pdf).

math.CV↗
arXiv2004

Four explicit formulas for the prolongations of an infinitesimal Lie symmetry and multivariate Faa di Bruno formulas

In 1979, building on S. Lie's theory of symmetries of (partial) differrential equations, P.J. Olver formulated inductive formulas which are appropriate for the computation of the prolongations of an infinitesimal Lie symmetry to jet spaces, for an arbitrary number n\geq 1 of independent variables (x^1, ..., x^n) and for an arbitrary number m \geq 1 of dependent variables (y^1, ..., y^m). This paper is devoted to elaborate a formalism based on multiple Kronecker symbols which enables one to handle these ``unmanageable'' prolongations and to discover the underlying complicated combinatorics. Proceeding progressively, we write down closed explicit formulas in four cases: n=m=1; n\geq 1, m=1; n=1, m\geq 1; general case n\geq 1, m\geq 1. As a subpart of the obtained formulas, we recover four possible versions of the (multivariate) Faà di Bruno formula. We do not employ the classical formalism based on the symmetric algebra ({\it cf.} e.g. H. Federer's book, p. 222), because it hides several explicit sums in symbolic compactifications, and because the presence of supplementary complexities ({\it e.g.} splitting of indices, combinatorics of partial derivatives) impedes us to apply such compactifications coherently. Our method of exposition is inductive: we conduct our reasonings by analyzing several thoroughly organized formulas, by comparing them together and by ``drifting'' towards generality, in homology with the classical style of L. Euler.

math.DG↗
arXiv2004

On removable singularities for CR functions in higher codimension

We establish the holomorphic wedge extendability of CR functions, defined on an everywhere locally minimal generic submanifold M of C^n and having singularities contained in a submanifold N of codimension 1, 2 or 3, assuming some transversality conditions about the relative disposition of N with respect to the complex tangent bundle to M. The statements hold in arbitrary codimension and are obtained by applying the theory of normal deformations of analytic discs, due to A.E. Tumanov in 1994 and also the FBI propagation of singularities phenomenon enjoyed by CR functions, due to J.-M. Trepreau in 1990. Related results in the hypersurface case were obtained simultaneously by B. Joricke in 1992-96 and by E. Porten in his thesis (1996).

math.CV↗
arXiv2004

Global minimality of generic manifolds and holomorphic extendibility of CR functions

Let M be a smooth generic submanifold of C^n. Tumanov showed that the direction of CR extendability parallel propagates with respect to a certain differential geometric partial connection in a quotient bundle of the normal bundle to M. M is said to be globally minimal at a point z in M if the CR orbit of z contains a neighborhood of z in M. It is shown that the vector space generated by the directions of CR-extendability of CR functions on M is preserved by the induced composed flow between two points in the same CR orbit. As an application, the main result of this paper, conjectured by J.-M. Trepreau in 1990, is established: for wedge extendability of CR functions to hold at every point in the CR-orbit of z M, it is sufficient that M be globally minimal at z.

math.CV↗