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Documents  Dutertre, Nicolas | enregistrements trouvés : 9

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Algebraic and Complex Geometry

Caustics appear in several areas in Physics (i.e., geometrical optics [10], the theory of underwater acoustic [2] and the theory of gravitational lensings [11], and so on) and Mathematics (i.e., classical differential geometry [12, 13] and the theory of differential equations [6, 7, 15], and so on). Originally the notion of caustics belongs to geometrical optics, which has strongly stimulated the study of singularities [14]. Their singularities are now understood as a special class of singularities, so called Lagrangian singularities [1, 16]. In this talk we start to describe the classical notion of evolutes (i.e., focal sets) in Euclidean plane (or, space) as caustics for understanding what are the caustics. The evolute is defined to be the envelope of the family of normal lines to a curve (or, a surface). The basic idea is that we may regard the normal line as a ray emanate from the curve (or, the surface), so that the evolute can be considered as a caustic in geometrical optics. Then we consider surfaces in Lorentz-Minkowski $3$-space and explain the direct analogy of the evolute (the Lorentzian evolute) of a timelike surface, whose singularities are the same as those of the evolute of a surface in Euclidean space generically. This case the normal lines of a timelike surface are spacelike, so these are not corresponding to rays in the physical sense. Therefore, the Lorentz evolute is not a caustic in the sense of geometric optics. In Lorentz-Minkowski $3$-space, the ray emanate from a spacelike curve is a normal line of the curve whose directer vector is lightlike, so the family of rays forms a lightlike surface (i.e., a light sheet). The set of critical values of the light sheet is called a lightlike focal curve along a spacelike curve. Actually, the notion of light sheets is important in Physics which provides models of several kinds of horizons in space-times [5]. On the other hand, a world sheet in a Lorentz-Minkowski $3$-space is a timelike surface consisting of a one-parameter family of spacelike curves. Each spacelike curve is called a momentary curve. We consider the family of lightlike surfaces along momentary curves in the world sheet. The locus of the singularities (the lightlike focal curves) of lightlike surfaces along momentary curves form a caustic. This construction is originally from the theoretical physics (the string theory, the brane world scenario, the cosmology, and so on) [3, 4]. Moreover, we have no notion of the time constant in the relativity theory. Hence everything that is moving depends on the time. Therefore, we consider world sheets in the relativity theory. In order to understand the situation easily, we only consider 2-dimensional world sheets in Lorentz-Minkowski $3$-space. We remark that we have results for higher dimensional cases and for other Lorentz space-forms similar to this special case [8, 9]. Caustics appear in several areas in Physics (i.e., geometrical optics [10], the theory of underwater acoustic [2] and the theory of gravitational lensings [11], and so on) and Mathematics (i.e., classical differential geometry [12, 13] and the theory of differential equations [6, 7, 15], and so on). Originally the notion of caustics belongs to geometrical optics, which has strongly stimulated the study of singularities [14]. Their singularities ...

53C40 ; 58K05

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Algebraic and Complex Geometry

Let $(V,p)$ be a complex isolated complete intersection singularity germ (an ICIS). It is well-known that its Milnor number $\mu$ can be expressed as the difference:
$$\mu = (-1)^n ({\rm Ind}_{GSV}(v;V) - {\rm Ind}_{rad}(v;V)) \;,$$
where $v$ is a continuous vector field on $V$ with an isolated singularity at $p$, the first of these indices is the GSV index and the latter is the Schwartz (or radial) index. This is independent of the choice of $v$.
In this talk we will review how this formula extends to compact varieties with non-isolated singularities. This depends on two different ways of extending the notion of Chern classes to singular varieties. On elf these are the Fulton-Johnson classes, whose 0-degree term coincides with the total GSV-Index, while the others are the Schwartz-McPherson classes, whose 0-degree term is the total radial index, and it coincides with the Euler characteristic. This yields to the well known notion of Milnor classes, which extend the Milnor number. We will discuss some geometric facts about the Milnor classes.
Let $(V,p)$ be a complex isolated complete intersection singularity germ (an ICIS). It is well-known that its Milnor number $\mu$ can be expressed as the difference:
$$\mu = (-1)^n ({\rm Ind}_{GSV}(v;V) - {\rm Ind}_{rad}(v;V)) \;,$$
where $v$ is a continuous vector field on $V$ with an isolated singularity at $p$, the first of these indices is the GSV index and the latter is the Schwartz (or radial) index. This is independent of the choice ...

32S65 ; 14B05 ; 57R20

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- 1065 p.
ISBN 978-981-270410-8

Localisation : Colloque 1er étage (MARS)

classe caractéristique # désingularisation # singularités de courbes planes # polytope # forme symplectique singulière # résolution de singularités # fibre de Milnor # surface singulière # variété torique

14-06 ; 32-06 ; 58-06 ; 00B25 ; 32Sxx

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Logic and Foundations

Diophantine properties of subsets of $\mathbb{R}^n$ definable in an o-minimal expansion of the ordered field of real numbers have been much studied over the last few years and several applications to purely number theoretic problems have been made. One line of inquiry attempts to characterise the set of definable functions $f : \mathbb{R} \to \mathbb{R}$ having the property that $f(\mathbb{N}) \subset \mathbb{N}$. For example, a result of Thomas, Jones and myself shows that if the structure under consideration is $\mathbb{R}_{exp}$ (the real field expanded by the exponential function) and if, for all positive $r, f(x)$ eventually grows more slowly than $exp(x^r)$, then $f$ is necessarily a polynomial with rational coefficients. In this talk I shall improve this result in two directions. Firstly, I take the structure to be $\mathbb{R}_{an,exp}$ (the expansion of $\mathbb{R}_{exp}$ by all real analytic functions defined on compact balls in $\mathbb{R}^n$) and secondly, I allow the growth rate to be $x^N \cdot 2^x$ for arbitrary (fixed) $N$. The conclusion is that $f(x) = p(x) \cdot 2^x + q(x)$ for sufficiently large $x$, where $p$ and $q$ are polynomials with rational coefficients.

I should mention that over ninety years ago Pólya established the same result for entire functions $f : \mathbb{C} \to \mathbb{C}$ and that in 2007 Langley weakened this assumption to $f$ being regular in a right half-plane of $\mathbb{C}$. I follow Langley’s method, but first we must consider which $\mathbb{R}_{an,exp}$-definable functions actually have complex continuations to a right half-plane and, as it turns out, which of them have a definable such continuation.
Diophantine properties of subsets of $\mathbb{R}^n$ definable in an o-minimal expansion of the ordered field of real numbers have been much studied over the last few years and several applications to purely number theoretic problems have been made. One line of inquiry attempts to characterise the set of definable functions $f : \mathbb{R} \to \mathbb{R}$ having the property that $f(\mathbb{N}) \subset \mathbb{N}$. For example, a result of ...

03C64 ; 26E05

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Logic and Foundations

The Pila-Wilkie theorem gives a bound on the number of rational points of bounded height lying on the transcendental part of a set definable in an o-minimal expansion of the real field. After discussing this result, I’ll describe various classes of curves for which the Pila-Wilkie bound can be improved. I’ll also give some examples and perhaps some applications.

03C64 ; 26E05

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Algebraic and Complex Geometry

Given a space curve, the surface ruled by tangent lines to the curve is called the tangent surface or the tangent developable to the curve. Tangent surfaces were studied by many mathemati- cians, Euler, Monge, Cayley, etc. The tangent surface has necessarily singularities along the original curve (curve of regression). The singularities are classified by Cleave, Mond, Arnold, Shcherbak and so on. In this talk we provide several generalisations of the known classification results. In particular we consider, in one direction, tangent surfaces to possibly singular curves in an ambient space of any dimension with any affine connection. In another direction, we study "abnormal" tangent surfaces to integral curves of a Cartan distribution in five space. The exposition will be performed via a generalised notion of "frontal". Given a space curve, the surface ruled by tangent lines to the curve is called the tangent surface or the tangent developable to the curve. Tangent surfaces were studied by many mathemati- cians, Euler, Monge, Cayley, etc. The tangent surface has necessarily singularities along the original curve (curve of regression). The singularities are classified by Cleave, Mond, Arnold, Shcherbak and so on. In this talk we provide several generalisations ...

53A20 ; 57R45 ; 58K40

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Algebraic and Complex Geometry

The Jacobian algebra, obtained from the ring of germs of functions modulo the partial derivatives of a function $f$ with an isolated singularity, has a non-degenerate bilinear form, Grothendieck Residue, for which multiplication by $f$ is a symmetric nilpotent operator. The vanishing cohomology of the Milnor Fibre has a bilinear form induced by cup product for which the nilpotent operator $N$, the logarithm of the unipotent part of the monodromy, is antisymmetric. Using the nilpotent operators we obtain primitive parts of the bilinear form and we compare both bilinear forms. In particular, over $\mathbb{R}$, we obtain signatures of these primitive forms, that we compare. The Jacobian algebra, obtained from the ring of germs of functions modulo the partial derivatives of a function $f$ with an isolated singularity, has a non-degenerate bilinear form, Grothendieck Residue, for which multiplication by $f$ is a symmetric nilpotent operator. The vanishing cohomology of the Milnor Fibre has a bilinear form induced by cup product for which the nilpotent operator $N$, the logarithm of the unipotent part of the ...

14B05 ; 32S65 ; 32S55

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Algebraic and Complex Geometry

Let $f$ be a homogeneous polynomial, defining a principal Zariski open set $D(f)$ in some complex projective space $\mathbb{P}^n$ and a Milnor fiber $F(f)$ in the affine space $\mathbb{C}^{n+1}$. Let $f_0, . . . , f_n$ denote the partial derivatives of $f$ with respect to $x_0, . . . , x_n$ and consider syzygies $a_0f_0 + a_1f1 + a_nf_n = 0$, where $a_j$ are homogeneous polynomials of the same degree $k$.
Using the mixed Hodge structure on $D(f)$ and $F(f)$, one can obtain information on the possible values of $k$.
Let $f$ be a homogeneous polynomial, defining a principal Zariski open set $D(f)$ in some complex projective space $\mathbb{P}^n$ and a Milnor fiber $F(f)$ in the affine space $\mathbb{C}^{n+1}$. Let $f_0, . . . , f_n$ denote the partial derivatives of $f$ with respect to $x_0, . . . , x_n$ and consider syzygies $a_0f_0 + a_1f1 + a_nf_n = 0$, where $a_j$ are homogeneous polynomials of the same degree $k$.
Using the mixed Hodge structure on ...

14B05 ; 13D02 ; 32S35

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Algebraic and Complex Geometry

Cohomology jump loci of local systems generalize the Milnor monodromy eigenvalues. We address recent progress on the local and global structure of cohomology jump loci. More generally, given an object with a notion of cohomology theory, how can one describe all its deformations subject to cohomology constraints? We give an answer in terms of differential graded Lie algebra pairs. This is joint work with Botong Wang.

14B05 ; 14F05

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