arXiv subjects

Dmitry Kerner

Publications and source records attributed to Dmitry Kerner.

At least 37 records · Page 2Linked to original sources

arXiv2017

Discriminant of the ordinary transversal singularity type. The local aspects

Consider a space X with the singular locus, Z=Sing(X), of positive dimension. Suppose both Z and X are locally complete intersections. The transversal type of X along Z is generically constant but at some points of Z it degenerates. We introduce (under certain conditions) the discriminant of the transversal type, a subscheme of Z, that reflects these degenerations whenever the generic transversal type is `ordinary'. The scheme structure of this discriminant is imposed by various compatibility properties and is often non-reduced. We establish the basic properties of this discriminant: it is a Cartier divisor in Z, functorial under base change, flat under some deformations of (X,Z), and compatible with pullback under some morphisms, etc. Furthermore, we study the local geometry of this discriminant, e.g. we compute its multiplicity at a point, and we obtain the resolution of its structure sheaf (as module on Z) and study the locally defining equation.

math.AG↗

arXiv2017

Durfee-type bound for some non-degenerate complete intersection singularities

The Milnor number, μ(X,0), and the singularity genus, p_g(X,0), are fundamental invariants of isolated hypersurface singularities (more generally, of local complete intersections). The long standing Durfee conjecture (and its generalization) predicted the inequality μ(X,0) \geq (n+1)!p_g(X,0), here n=dim(X,0). Recently we have constructed counterexamples, proposed a corrected bound and verified it for the homogeneous complete intersections. In the current paper we treat the case of germs with Newton-non-degenerate principal part when the Newton diagrams are "large enough", i.e. they are large multiples of some other diagrams. In the case of local complete intersections we prove the corrected inequality, while in the hypersurface case we prove an even stronger inequality.

math.AG↗

arXiv2017

A generalized FKG-inequality for compositions

We prove a Fortuin-Kasteleyn-Ginibre-type inequality for the lattice of compositions of the integer n with at most r parts. As an immediate application we get a wide generalization of the classical Alexandrov-Fenchel inequality for mixed volumes and of Teissier's inequality for mixed covolumes.

math.AC↗

arXiv2017

Lipschitz Normal Embeddings in the Space of Matrices

The germ of an algebraic variety is naturally equipped with two different metrics up to bilipschitz equivalence. The inner metric and the outer metric. One calls a germ of a variety Lipschitz normally embedded if the two metrics are bilipschitz equivalent. In this article we prove Lipschitz normal embeddedness of some algebraic subsets of the space of matrices. These include the space $m \times n$ matrices, symmetric matrices and skew-symmetric matrices of rank equal to a given number and their closures, and the upper triangular matrices with determinant $0$. We also make a short discussion about generalizing these results to determinantal varieties in real and complex spaces.

math.AG↗

arXiv2016

Group actions on filtered modules and finite determinacy. Finding large submodules in the orbit by linearization

Fix a module M over a local ring R and a group action G on M, not necessarily R-linear. To understand how large is the G-orbit of an element z\in M one looks for the large submodules of M lying in Gz. We provide the corresponding (necessary/sufficient) conditions in terms of the tangent space to the orbit, T_{(Gz,z)}. This question originates from the classical finite determinacy problem of Singularity Theory. Our treatment is rather general, in particular we extend the classical criteria of Mather (and many others) to a broad class of rings, modules and group actions. When a particular `deformation space' is prescribed, Σ\subseteq M, the determinacy question is translated into the properties of the tangent spaces, T_{(Gz,z)}, T_{(\Si,z)}, and in particular to the annihilator of their quotient.

math.AG↗

arXiv2016

Determinantal variety and normal embedding

The space of matrices of positive determinant GL^+_n inherits an extrinsic metric space structure from R^{n^2}. On the other hand, taking the infimum of the lengths of all paths connecting two points in GL^+_n gives an intrinsic metric. We prove bilipschitz equivalence for intrinsic and extrinsic metrics on GL^+_n, exploiting the conical structure of the stratification of the space of n by n matrices by rank.

math.AG↗

arXiv2016

Finite determinacy of matrices over local rings.II. Tangent modules to the miniversal deformations for group-actions involving the ring automorphisms

We consider matrices with entries in a local ring, Mat(m,n;R). Fix an action of group G on Mat(m,n;R), and a subset of allowed deformations, Σin Mat(m,n;R). The standard question (along the lines of Singularity Theory) is the finite-(Σ,G)-determinacy of matrices. In our previous work this determinacy question was reduced to the study of the tangent spaces to Σand to the orbit, T_{(Σ,A)}, T_{(GA,A)}, and their quotient: the tangent module to the miniversal deformation. In particular, the order of determinacy is controlled by the annihilator of this tangent module. Then we have studied this tangent module for the group action GL(m,R)\times GL(n,R) on Mat(m,n;R) and for various natural subgroups of it. These are R-linear group actions. In the current work we study this tangent module for group actions that involve the automorphisms of the ring, or, geometrically, group-actions that involve the local coordinate changes. (These actions are not R-linear.) We obtain various bounds on the support of this module. This gives ready-to-use criteria of determinacy for matrices, (embedded) modules and (skew-)symmetric forms.

math.AG↗

arXiv2016

A strong version of implicit function theorem

We suggest the necessary/sufficient criteria for the existence of a (order-by-order) solution y(x) of a functional equation F(x,y)=0 over a ring. In full generality, the criteria hold in the category of filtered groups, this includes the wide class of modules over (commutative, associative) rings. The classical implicit function theorem and its strengthening obtained by Tougeron and Fisher appear to be (weaker) particular forms of the general criterion. We obtain a special criterion for solvability of the equations arising from group actions, g(w)=w+u, here u is "small". As an immediate application we re-derive the classical criteria of determinacy, in terms of the tangent space to the orbit. Finally, we prove the Artin-Tougeron-type approximation theorem: if a system of C^\infty-equations has a formal solution and the derivative satisfies a Lojasiewicz-type condition then the system has a C^\infty-solution.

math.AC↗

arXiv2014

Block-diagonalization of matrices over local rings.II

Consider rectangular matrices over a local ring R. In the previous work we have obtained criteria for block-diagonalization of such matrices, i.e. U A V=A_1\oplus A_2, where U,V are invertible matrices over R. In this short note we extend the criteria to the decomposability of quiver representations over R.

math.RT↗

arXiv2014

Recombination formulae for the spectrum of curve singularities and some applications

We obtain some recombination formulae for the spectra of (complex, reduced) plane curve singularities. As an application we prove: a generalization of Durfee's bound; a generalization of Givental's bound; the multiplicity of the curve singularity is determined by its spectrum; for many curve singularities all the multiplicities of exceptional divisors of the resolution are determined by the spectrum; etc.

math.AG↗

arXiv2014

On varieties of Lie algebras of maximal class

We study complex projective varieties that parametrize (finite-dimensional) filiform Lie algebras over C, using equations derived by Millionshchikov. In the infinite-dimensional case we concentrate our attention on N-graded Lie algebras of maximal class. As shown by A. Fialowski (see also [shalev:97], [millionshchikov:04]) there are only three isomorphism types of N-graded Lie algebras $L=\oplus^{\infty}_{i=1} L_i$ of maximal class generated by L_1 and L_2, L= . Vergne described the structure of these algebras with the property L= . In this paper we study those generated by the first and q-th components where q>2, L= . Under some technical condition, there can only be one isomorphism type of such algebras. For q=3 we fully classify them. This gives a partial answer to a question posed by Millionshchikov.

math.RT↗

arXiv2014

Many singularities are not stably equivalent to Newton-non-degenerate singularities

This paper has been withdrawn. Consider an isolated complex hypersurface singularity, f(x_1,..,x_n)=0. For Newton-non-degenerate singularities the local topology is completely determined by an associated polyhedral object, the Newton diagram. "Most" singularities are not Newton-non-degenerate, for any choice of local coordinates. An old question of Arnol'd asks whether for any hypersurface singularity there exists a stabilization, f(x_1,...,x_n)+z^2_1+...+z^2_r, that becomes Newton-non-degenerate after some change of coordinates. The answer is: "totally no". We give some simple obstructions and present particular examples of plane curve singularities that have no Newton-non-degenerate stabilization (in any local coordinates).

math.AG↗

arXiv2012

The 'corrected Durfee's inequality' for homogeneous complete intersections

We address the conjecture of [Durfee1978], bounding the singularity genus, p_g, by a multiple of the Milnor number, μ, for an n-dimensional isolated complete intersection singularity. We show that the original conjecture of Durfee, namely (n+1)!p_g\leq μ, fails whenever the codimension r is greater than one. Moreover, we propose a new inequality, and we verify it for homogeneous complete intersections. In the homogeneous case the inequality is guided by a `combinatorial inequality', that might have an independent interest.

math.AG↗

arXiv2012

Determinantal representations of singular hypersurfaces in P^n

A (global) determinantal representation of hypersurface in P^n is a matrix, whose entries are linear forms in homogeneous coordinates and whose determinant defines the hypersurface. We study the properties of such representations for singular (possibly reducible or non-reduced) hypersurfaces. In particular, we obtain the decomposability criteria for determinantal representations of globally reducible hypersurfaces. Further, we classify the determinantal representations in terms of the corresponding kernel sheaves on $X$. Finally, we extend the results to the case of symmetric/self-adjoint representations, with implications to hyperbolic polynomials and generalized Lax conjecture.

math.AG↗

arXiv2011

Decomposability of local determinantal representations of hypersurfaces

Let M be a matrix whose entries are power series in several variables and determinant det(M) does not vanish identically. The equation det(M)=0 defines a hypersurface singularity and the (co)-kernel of M is a maximally Cohen-Macaulay module over the local ring of this singularity. Suppose the determinant det(M) is reducible, i.e. the hypersurface is locally reducible. A natural question is whether the matrix is equivalent to a block-diagonal or at least to an upper-block-triangular. (Or whether the corresponding module is decomposable or at least is an extension.) We give various necessary and sufficient criteria. Two classes of such matrices of functions appear naturally in the study of decomposability: those with many generators (e.g. maximally generated or Ulrich maximal) and those that descend from birational modifications of the hypersurface by pushforwards (i.e. correspond to modules over bigger rings). Their properties are studied.

math.AG↗

arXiv2011

A counterexample to Durfee conjecture

An old conjecture of Durfee 1978 bounds the ratio of two basic invariants of complex isolated complete intersection surface singularities: the Milnor number and the singularity (or geometric) genus. We give a counterexample for the case of non-hypersurface complete intersections, and we formulate a weaker conjecture valid in arbitrary dimension and codimension. This weaker bound is asymptotically sharp. In this note we support the validity of the new proposed inequality by its verification in certain (homogeneous) cases. In our subsequent paper we will prove it for several other cases and we will provide a more comprehensive discussion.

math.AG↗

arXiv2010

Normal forms of matrices over the ring of formal series

Matrices over the ring of formal power series are considered. Normal forms with respect to various sub-groups of the two-sided transformations are constructed. The construction is based on the special property of the action: it induces a filtration by projectors on sub-spaces of polynomial maps.

math.RT↗

arXiv2010

The Milnor fibre signature is not semi-continuous

Consider the germ of an isolated surface singularity in $(\mC^3,0)$. The corresponding Milnor fibre possesses the homology lattice (the integral middle homology with a natural symmetric intersection form). An old question of A.Durfee (1978) asks: is the signature of this form non-increasing under degenerations? The present article answers negatively: We give examples of Newton non-degenerate families where the signature increases under degeneration.

math.AG↗