Remarks on Normal Generation of Special Line Bundles on Algebraic Curves

Let L be a very ample line bundle with h ¹(L) ≥2 on a curve of genus g. We prove that L is normally generated if deg(L) ≥2g ? 1 ? 4h ¹(L) for large enough genus g.     

BLOWING UP PROJECTIVELY NORMAL SPACE CURVES AND VERY AMPLE DIVISORS

Let C ? P ³ be a projectively normal curve and π : X (C) → P ³ the blowing-up of C,E the exceptional divisor of π and H ∈(X (C)) the total transform of the hyperplane line bundle. Let H (C,t) be the Hilbert function of C and Δ² H its second difference function. Let σ be the minimal integer such that Δ² H (C σ) = 0. Here we prove that is C has no σ-secant line, then |σ H – E| is very ample.     

The fundamental group of a triangular algebra without double bypasses

Let A be a basic connected finite dimensional algebra over a field of characteristic zero. A fundamental group depending on the presentation of A has been defined by several authors [see R. Martínez-Villa, J.A. de La Peña, The universal cover of a quiver with relations, J. Pure Appl. Algebra 30 (1983) 277–292]. Assuming the quiver of A has no oriented cycles and no double bypasses, we show there exists a suitable presentation of A with quiver and admissible relations, with fundamental group denoted by ${\pi }_1(A)$, such that the fundamental group of any other presentation of A with quiver and admissible relations is a quotient of ${\pi }_1(A)$. To cite this article: P. Le Meur, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

D'Alembert's proof of the fundamental theorem of algebra

D'Alembert's proof of the fundamental theorem of algebra (FTA), the first published, is still widely misunderstood. Typical of d'Alembert, his work is bold and imaginative but in need of significant repair. The proof is examined in detail, in both the 1746 and 1754 versions, along with commentary over 250 years and recent efforts to revive d'Alembert's reputation. A particular challenge is to work with algebraic equations while avoiding dependence on the FTA itself. A repaired version is offered.     

Projectively normal embeddings of an algebraic curve with a net

Let $X$ be a smooth irreducible algebraic curve of genus $g$. The projective normality of a complete embedding of $X$ is determined by only its quadratic normality in case the embedding is of degree at least $g$. This means that the complete embedding fails to be projectively normal if and only if it admits an effective divisor which fails to impose independent conditions on quadrics in the embedded projective space. Thus if $X$ admits a net, then it is interesting to compare the conditions for the projective normality of an embedding of $X$ with properties of conic sections of the plane curve given by the net. For such a curve $X$ with a net, we show that the projectively normal embeddings are closely related to properties of conic sections.     

Abduction-induction (generalization) processes of elementary majors on figural patterns in algebra

The article deals with issues concerning the abductive-inductive reasoning of 42 preservice elementary majors on patterns that consist of figural and numerical cues. We discuss: ways in which the participants develop generalizations about classes of abstract objects; abductive processes they exhibit which support their induction leading to a generalization; ways they justify their generalizations in the abductive stage, and; the effects of figural and numerical cues in the manner they construct a plausible abductive generalization. Two types of abductions are explored, model-based and manipulative. A proposed abductive-inductive reasoning process for pattern sequences is presented and discussed in the concluding section.     

Secant planes of projective curves embedded by nonspecial line bundles

For a nonspecial line bundle ? on a smooth curve X we consider a presentation ??𝒦_X?D+E which is minimal with respect to deg E. If ? is very ample, then this minimality means that any n-points of φ_?(X) with n≤deg E?1 are in general position while φ_?(E) spans a (deg E?2)-plane. In this work, we investigate conditions on D and E for ??𝒦_X?D+E to be minimal. We also observe s-secant (s?k?1)-planes which are minimal with respect to the secant degree s for a given k. We apply minimal presentations to problems about the exactness of Green-Lazarsfeld’s conjecture on property (N_p).     

The fundamental groups of one-dimensional wild spaces and the Hawaiian earring

Let be a one-dimensional space which contains a copy of a circle and let it not be semi-locally simply connected at any point on Then the fundamental group of cannot be embeddable into a free -product of n-slender groups, for instance, the fundamental group of the Hawaiian earring. Consequently, any one of the fundamental groups of the Sierpinski gasket, the Sierpinski curve, and the Menger curve is not embeddable into the fundamental group of the Hawaiian earring.

     

Linear series on a curve of compact type bridged by a chain of elliptic curves

In the present paper we investigate conditions for the non-existence of a smoothable limit linear series on a curve of compact type such that two smooth curves are bridged by a chain of two elliptic curves. Combining this work with results on the existence of a smoothable limit linear series on such a curve, we show relations among Brill–Noether loci of codimension at most two in the moduli space of complex curves. Specifically, Brill–Noether loci of codimension two have mutually distinct supports.     

The fundamental theorem of algebra for Hamilton and Cayley numbers

In this paper we prove the fundamental theorem of algebra for polynomials with coefficients in the skew field of Hamilton numbers (quaternions) and in the division algebra of Cayley numbers (octonions). The proof, inspired by recent definitions and results on regular functions of a quaternionic and of a octonionic variable, follows the guidelines of the classical topological argument due to Gauss. G. Gentili and F. Vlacci are partially supported by G.N.S.A.G.A. of the I.N.D.A.M. and by M.I.U.R.     

On certain linearized polynomials with high degree and kernel of small dimension

Let f be the ${\mathbb{F}}_q$-linear map over ${\mathbb{F}}_{q^{2n}}$ defined by $x?x+a{x}^{q^s}+b{x}^{q^{n+s}}$ with $\gcd ?(n,s)=1$. It is known that the kernel of f has dimension at most 2, as proved by Csajbók et al. in [9]. For n big enough, e.g. $n\ge 5$ when $s=1$, we classify the values of $b/a$ such that the kernel of f has dimension at most 1. To this aim, we translate the problem into the study of some algebraic curves of small degree with respect to the degree of f; this allows to use intersection theory and function field theory together with the Hasse-Weil bound. Our result implies a non-scatteredness result for certain high degree scattered binomials, and the asymptotic classification of a family of rank metric codes.     

Exponential iterated integrals and the relative solvable completion of the fundamental group of a manifold

We develop a class of integrals on a manifold M called exponential iterated integrals, an extension of K.T. Chen's iterated integrals. It is shown that the matrix entries of any upper triangular representation of π₁(M,x) can be expressed via these new integrals. The ring of exponential iterated integrals contains the coordinate rings for a class of universal representations, called the relative solvable completions of π₁(M,x). We consider exponential iterated integrals in the particular case of fibered knot complements, where the fundamental group always has a faithful relative solvable completion.     

The stack of formal groups in stable homotopy theory

We construct the algebraic stack of formal groups and use it to provide a new perspective onto a recent result of M. Hovey and N. Strickland on comodule categories for Landweber exact algebras. This leads to a geometric understanding of their results as well as to a generalisation.     

Monoidal functors on the category of representations of a triangular Hopf algebra

Let (H,R) be a triangular Hopf algebra. The monoidal functors on the category of representations ofH is studied, and a universal quantum commutative algebraSeR(M) and a dual H°-comoduleM° for any H-moduleM with an integrale are constructed. Both constructions given here have tensor isomorphism properties. Project supported by the National Natural Science Foundation of China.