Ragni Piene

• Dickenstein, Alicia & Piene, Ragni (2017). Higher order selfdual toric varieties. Annali di Matematica Pura ed Applicata.  ISSN 0373-3114.  196(5), s 1759- 1777 . doi: 10.1007/s10231-017-0637-4 Full text in Research Archive. Show summary

  The notion of higher order dual varieties of a projective variety, introduced in \cite{P83}, is a natural generalization of the classical notion of projective duality. In this paper we present geometric and combinatorial characterizations of those equivariant projective toric embeddings that satisfy higher order selfduality. We also give several examples and general constructions. In particular, we highlight the relation with Cayley-Bacharach questions and with Cayley configurations.

• Piene, Ragni (2016). Chern–Mather classes of toric varieties. arXiv.org.  ISSN 2331-8422. Show summary

  The purpose of this short note is to prove a formula for the Chern--Mather classes of a toric variety in terms of its orbits and the local Euler obstructions at general points of each orbit (Theorem 2). We use the general definition of the Chern--Schwartz--MacPherson classes (see \cite{MR0361141}) and their special expression in case of a toric variety (see \cite{MR1197235}). As a corollary, we obtain a formula by Matsui--Takeuchi \cite[Corollary 1.6]{MR2737807}. Alternatively, one could deduce the formula of Theorem \ref{CM} from the Matsui--Takeuchi formula, by using our general result \cite[Th\'eor\`eme 3]{MR1074588} for the degree of the polar varieties in terms of the Chern--Mather classes.

• Lanteri, Antonio; Mallavibarrena, Raquel & Piene, Ragni (2015). Inflectional loci of quadric fibrations. Journal of Algebra.  ISSN 0021-8693.  441, s 363- 397 . doi: 10.1016/j.jalgebra.2015.06.023 Full text in Research Archive. Show summary

  Quadric fibrations over smooth curves are investigated with respect to their osculatory behavior. In particular, bounds for the dimensions of the osculating spaces are determined, and explicit formulas for the classes of the inflectional loci are exhibited under appropriate assumptions. Moreover, a precise description of the inflectional loci is provided in several cases. The associated projective bundle and its image in the ambient projective space of the quadric fibration, the enveloping ruled variety, play a significant role. Several examples are discussed to illustrate concretely the various situations arising in the analysis.

• Piene, Ragni (2015). Discriminants, polytopes, and toric geometry, In Pierre Cartier; A. D. R. Choudary & Michel Waldschmidt (ed.),  Mathematics in the 21st Century.  Springer Science+Business Media B.V..  ISBN 978-3-0348-0858-3.  Chapter.  s 151 - 162 Show summary

  Toric varieties can be considered as a meeting point of algebra, geometry and combinatorics. Though they constitute only a small subset of all algebraic varieties, their structure is rich enough to make them interesting when formulating and testing conjectures. Because of their combinatorial description, results in algebraic geometry can some times be proved using combinatorics, and combinatorial results can sometimes be proved by algebraic-geometric methods. In this note we will survey results by several authors concerning the description of dual and higher order dual varieties of toric varieties in terms of their associated lattice polytopes. Toric varieties that are ruled correspond to Cayley polytopes, and we will describe several properties and characterizations of Cayley polytopes using toric geometry.

• Piene, Ragni (2015). Polar varieties revisited, In Jaime Gutierrez; Josef Schicho & Martin Weimann (ed.),  Computer Algebra and Polynomials.  Springer Science+Business Media B.V..  ISBN 978-3-319-15080-2.  Chapter.  s 139 - 150 Show summary

  We recall the definition of classical polar varieties, as well as those of affine and projective reciprocal polar varieties. The latter are defined with respect to a non-degenerate quadric, which gives us a notion of orthogonality. In particular we relate the reciprocal polar varieties to the ``Euclidean geometry'' in projective space. The Euclidean distance degree and the degree of the focal loci can be expressed in terms of the ranks, i.e., the degrees of the classical polar varieties, and hence these characters can be found also for singular varieties, when one can express the ranks in terms of the singularities.

• Dickenstein, Alicia; Di Rocco, Sandra & Piene, Ragni (2014). Higher order duality and toric embeddings. Annales de l'Institut Fourier.  ISSN 0373-0956.  64(1), s 375- 400 . doi: 10.5802/aif.2851 Show summary

  ABSTRACT. The notion of higher order dual varieties of a projective variety, introduced by Piene in 1983, is a natural generalization of the classical notion of projective duality. In this paper we study higher order dual varieties of projective toric embeddings. We compute the degree of the second dual variety of a 2-jet spanned projective embedding of a smooth toric threefold in geometric and combinatorial terms, and we classify those whose second dual variety has dimension less than expected. We also describe the tropicalization of the k-th dual variety of an equivariantly embedded (not necessarily normal) toric variety.

• Piene, Ragni (2014). Algebraic spline geometry: some remarks, In Tor Dokken & Agnar Georg Peder Muntingh (ed.),  SAGA – Advances in ShApes, Geometry, and Algebra. Results from the Marie Curie Initial Training Network.  Springer.  ISBN 978-3-319-08634-7.  Kapittel 9.  s 169 - 175
• Lanteri, Antonio; Mallavibarrena, Raquel & Piene, Ragni (2012). Inflectional Loci of Scrolls over Smooth, Projective Varieties. Indiana University Mathematics Journal.  ISSN 0022-2518.  61(2), s 717- 750
• Kleiman, Steven; Piene, Ragni & Tyomkin, Ilya (2011). Enriques diagrams, arbitrarily near points, and Hilbert schemes. Rendiconti Lincei - Matematica e Applicazioni.  ISSN 1120-6330.  22(4), s 411- 451 . doi: 10.4171/RLM/608
• Dickenstein, A; Di Rocco, S & Piene, Ragni (2009). Classifying smooth lattice polytopes via toric fibrations. Advances in Mathematics.  ISSN 0001-8708.  222(1), s 240- 254 . doi: 10.1016/j.aim.2009.04.002
• Johansen, Pål Hermunn; Piene, Ragni & Løberg, Magnus Bjørnsen (2008). Monoid hypersurfaces, In Bert Jüttler & Ragni Piene (ed.),  Geometric Modeling and Algebraic Geometry.  Springer.  ISBN 978-3-540-72184-0.  Kapittel.  s 55 - 77
• Lanteri, Antonio; Mallavibarrena, Raquel & Piene, Ragni (2008). Inflectional loci of scrolls. Mathematische Zeitschrift.  ISSN 0025-5874.  258, s 557- 564 . doi: 10.1007/s00209-007-0185-5
• Mork, Heidi Camilla & Piene, Ragni (2008). Polars of real singular plane curves, In Alicia Dickenstein; Frank Olaf Schreyer & Andrew J. Sommese (ed.),  Algorithms in Algebraic Geometry.  Springer Science+Business Media B.V..  ISBN 978-0-387-75154-2.  Kapittel.  s 99 - 115
• Piene, Ragni (2005). Singularities of some projective rational surfaces, In Tor Dokken (ed.),  Computational Methods for Algebraic Spline Surfaces.  Springer.  ISBN 3-540-23274-5.  Article.  s 171 - 182
• Kleiman, Steven & Piene, Ragni (2004). Node polynomials for families: methods and applications. Mathematische Nachrichten.  ISSN 0025-584X.  271, s 69- 90 . doi: 10.1002/mana.200310182 Show summary

  We continue the development of methods for enumerating nodal curves on smooth complex surfaces, extending the range of validity. We apply the new methods in three important cases. First, for up to eight nodes, we prove Göttsche's conjecture about plane curves of low degree. Second, we prove Vainsencher's conjectural enumeration of irreducible six-nodal plane curves on a general quintic threefold in four-space, which is important for Clemens' conjecture and mirror symmetry. Third, we supplement Bryan and Leung's enumeration of nodal curves in a given homology class on an Abelian surface of Picard number 1.

• Kleiman, Steven & Piene, Ragni (2001). Node polynomials for families: results and examples. arXiv. Show summary

  We continue the development of methods for enumerating nodal curves on smooth complex surfaces, stressing the range of validity. We illustrate the new methods in three important examples. First, for up to eight nodes, we confirm G\"ottsche's conjecture about plane curves of low degree. Second, we justify Vainsencher's enumeration of irreducible six-nodal plane curves on a general quintic threefold in four-space. Third, we supplement Bryan and Leung's enumeration of nodal curves in a given homology class on an Abelian surface of Picard number one.

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• Holden, Helge & Piene, Ragni (ed.) (2014). The Abel Prize 2008–2012. Springer Science+Business Media B.V..  ISBN 978-3-642-39448-5.  571 s.
• Holden, Helge & Piene, Ragni (ed.) (2010). The Abel Prize 2003-2007. The First Five Years. Springer Publishing Company.  ISBN 978-3-642-01372-0.  327 s.
• Jüttler, Bert & Piene, Ragni (ed.) (2008). Geometric Modeling and Algebraic Geometry. Springer.  ISBN 978-3-540-72184-0.  232 s.
• Piene, Ragni; Elkadi, Mohamed & Mourrain, Bernard (ed.) (2006). Algebraic geometry and geometric modeling. Springer.  ISBN 978-3-540-33274-9.  252 s.
• Laudal, Olav Arnfinn & Piene, Ragni (ed.) (2004). The Legacy of Niels Henrik Abel. Springer.  ISBN 3-540-43826-2.  784 s.

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• Persson, Ulf & Piene, Ragni (2018, 15. mars). Interview with Ragni Piene. [Tidsskrift].  EMS Newsletter.
• Piene, Ragni (2017). Algebraic geometry for geometric modeling. Show summary

  To model an object, one can try to get an approximate the shape by putting together simple geometric objects like line segments and plane patches. To allow for curved shapes, one needs different objects -- the simplest ones being pieces of conics or spheres or cylinders, so-called natural quadrics. With increased computer hardware and software, it is possible to go further and use objects defined by polynomials of higher degree.

• Piene, Ragni (2017). Euclidean projective geometry, reciprocal polar varieties, and focal loci.. Show summary

  We can regard an affine space as projective space minus a hyperplane at infinity, together with a notion of perpendicularity in that hyperplane. This gives a “Euclidean” structure on the affine space, including a Euclidean normal bundle of a given variety. This is used to define the reciprocal polar varieties, the end point map, and the focal loci. The degrees of the reciprocal polar varieties can be expressed in terms of the degrees of the ordinary polar varieties, also in the case that the variety is singular. There has been recent interest in computing, or bounding, these degrees. I will give several examples, especially in the case of curves, surfaces, and toric varieties.

• Piene, Ragni (2017). Euclidean projective geometry: reciprocal polar varieties and focal loci. Show summary

  Euclidean orthogonality and ``distance'' can be defined in a given (real or complex) projective space with respect to a quadric, or a quadric in the hyperplane at infinity. Using this, one defines the Euclidean normal bundle and reciprocal polar varieties of a given, possibly singular, projective variety. These polar varieties are related to the classical concepts of focal loci and caustics of reflections. In fact, the focal locus is the branch locus of the end point map of the Euclidean normal bundle. There has been recent interest in computing, or bounding, the degree of the focal locus. I will give examples of such computations, especially in the case of curves, surfaces, and toric varieties.

• Piene, Ragni (2017). Polar varieties and Euclidean distance degree. Show summary

  Reciprocal polar varieties of a given, possibly singular, projective variety are defined with respect to a non-degenerate quadric hypersurface. The quadric induces a notion of orthogonality in the projective space, hence also a notion of ``Euclidean geometry''. Thus one can define the Euclidean distance degree and the Euclidean normal bundle, as well as the classical concepts of focal loci and caustics of reflections. The Euclidean distance degree and the degree of the focal locus can be expressed in terms of the degrees of the classical polar varieties. I will give some examples in the case of curves, surfaces, and toric varieties.

• Piene, Ragni (2016). Algebraic splines and generalized Stanley–Reisner rings. Show summary

  Given a simplicial complex $\Delta\subset \mathbb R^d$, let $C^r_k(\Delta)$ denote the vector space of piecewise polynomial functions (algebraic splines) of degree $\le k$ and smoothness $r$. A major problem is to determine the dimension (and construct bases) of these vector spaces. Pioneering work by Billera, Rose, Schenck, and others gave upper and lower bounds using homological methods. The ring of continuous splines $C^0(\Delta)=C^0_k(\Delta)$ is (essentially) equal to the face ring, or Stanley--Reisner ring, of $\Delta$ and has the property that its geometric realization describes $\Delta$. More precisely, the part of ${\rm Spec}(C^0(\Delta))$ lying in a certain hyperplane and having nonnegative coordinates is ``equal'' to $\Delta$. Here we shall consider the \emph{generalized Stanley--Reisner rings} $C^r(\Delta):=\oplus_k C^r_k(\Delta)\subset C^0(\Delta)$. We present a conjectural description of ${\rm Spec}(C^r(\Delta))$ generalizing the one for $r=0$. To illustrate the conjecture, some very simple examples will be given.

• Piene, Ragni (2016). Algebraic splines and generalized Stanley–Reisner rings. Show summary

  Given a simplicial complex $\Delta\subset \mathbb R^d$, let $C^r_k(\Delta)$ denote the vector space of piecewise polynomial functions (algebraic splines) of degree $\le k$ and smoothness $r$. A major problem is to determine the dimension of these vector spaces. Pioneering work by Billera, Rose, Schenck, and others give upper and lower bounds using homological methods. Here we shall consider the rings $C^r(\Delta):=\oplus_k C^r_k(\Delta)$ that we shall call the generalized Stanley--Reisner rings of $\Delta$. The ring of continuous splines $C^0(\Delta)$ is (essentially) the face ring of $\Delta$ and has the property that its geometric realization describes $\Delta$. More precisely, the part of ${\rm Spec}(C^0(\Delta)$ lying in a certain hyperplane and having nonnegative coordinates ``is'' $\Delta$. I propose a conjectural generalization of this situation, giving a description of ${\rm Spec}(C^r(\Delta))$ for $r\ge 0$. To support the conjecture, some very simple examples will be given.

• Piene, Ragni (2016). Enumeration of singular curves on surfaces. Show summary

  The number of $r$-nodal curves in a fixed linear system on a smooth surface passing through the appropriate number of points is given by a polynomial in the Chern numbers of the surface. There are various generalizations of this enumerative question, e.g. to algebraic systems of curves on algebraic families of surfaces, or to curves with other singularities than nodes. Another generalization is to consider the case of singular surfaces. One could hope that one could obtain the corresponding formulas by replacing the Chern classes of the smooth surface with the Chern--Mather classes of the singular surface. Since for toric surfaces the Chern--Mather classes are known -- they are equal to the Chern--Schwartz--MacPherson classes weighted by the local Euler obstruction -- toric surfaces provide a good testing ground. However, as shown by Liu--Osserman, who considered some special toric surfaces, the situation is not that simple. In the talk I will discuss these problems, as well as some of the different approaches to solving them and understanding their generating functions.

• Piene, Ragni (2015). Counting curves on singular surfaces. Show summary

  Let $S$ be a smooth projective surface and $\mathcal L$ a line bundle. As is now well known, the number of $r$-nodal curves in the linear system $|\mathcal L|$ passing through he appropriate number of points on $S$ can be expressed as a polynomial of degree $r$ in the Chern numbers $\mathcal L^2$, $K_S\cdot \mathcal L$, $K_S^2$, and $c_2(S)$. There has recently been works by several authors (Ardila--Block, Liu--Osserman, Block--G\"ottsche) that attempt to find similar formulas in the case that $S$ is a singular toric surface. I will discuss this work, and also initial recent work by N\o dland in the case of weighted projective planes.

• Piene, Ragni (2015). Higher order self-dual toric varieties. Show summary

  To a projective variety one associates its dual variety: the set of hyperplanes tangent to the given variety. It is well known that the only smooth varieties that are ``self-dual'', i.e., are such that the dual variety is isomorphic to the variety, are quadric hypersurfaces in characteristic 0 and Fermat hypersurfaces in characteristic $p>0$. By replacing tangency conditions by higher order contact conditions, one can define \emph{higher order} dual varieties and ask for a classification of those that are ``higher'' self-dual. In this talk I will report on joint work with Alicia Dickenstein, where we study higher order dual varieties of \emph{toric embeddings}. A lattice point configuration $\mathcal A \subset \mathbb Z^n$ defines a (real or complex) toric embedding in $\mathbb P^N$, where $N=\# \mathcal A -1$. The aim is to characterize those varieties that are isomorphic to one of its higher order dual varieties, in particular to find conditions on the configuration $\mathcal A$ or its convex lattice polytope $P={\rm Conv}(\mathcal A)$ for this to happen. I will explain our results, give examples, and state some conjectures.

• Piene, Ragni (2015). Projective geometry from a toric point of view. Show summary

  Classical projective geometry addresses questions like classification, duality, divisors, sections and projections, enumerative geometry, etc. Although projective \emph{toric} varieties form but a small subset of all projective varieties, they constitute nonetheless a rich and interesting playground for the study of such questions. The fact that there is a ``dictionary'' between toric projective varieties and lattice point configurations and convex lattice polytopes, makes it possible to use combinatorial methods to prove algebraic geometric results, and vice versa. In this talk I will survey some recent results by various authors, especially concerning the classification of ``hollow'' polytopes and the characterization of selfdual and higher order selfdual toric varieties. In both cases, so-called Cayley polytopes play a major role.

• Jemterud, Torkild & Piene, Ragni (2014, 26. august). Ekko. [Radio].  NRK.
• Piene, Ragni (2014). An interview with Cheryl Praeger. Seoul Intelligencer.  s 16- 18
• Piene, Ragni (2014). Higher order osculating spaces and dual varieties of toric varieties. Show summary

  The dual varieties of a projective variety can be studied via the conormal varieties lying in the point-hyperplane incidence variety. In the case the variety is a toric embedding, properties of the dual varieties can be translated into properties of the lattice point configuration or polytope. In particular, one can give combinatorial criteria for such a variety to be dual defective or (higher) selfdual. Several examples, including Cayley configurations and rational normal scrolls, will be given. Connections with diophantine problems will be highlighted. This is joint work with Alicia Dickenstein.

• Piene, Ragni & Næss Olsen, Maren (2014, 21. februar). Mysteriet med kvinnene som forsvant.  Morgenbladet.
• Moe, Torgunn Karoline; Piene, Ragni & Ranestad, Kristian (2013). Cuspidal curves on Hirzebruch surfaces.
• Piene, Ragni (2013). Discriminants, polytopes and toric geometry. Show summary

  Toric varieties play an important role at the crossroad of algebra, geometry and combinatorics. Toric geometry is both rigid and rich, and allows for testing conjectures and proving results in algebraic geometry using combinatorial methods, and in combinatorics using algebro-geometrical methods. An example of the latter is a combinatorial characterization of certain Cayley polytopes. This talk will survey recent work by various authors concerning the description and characterization of dual and higher order dual varieties of projective toric varieties in terms of the corresponding lattice configurations and polytopes. In particular we will characterize lattice configurations which give self dual, or higher self dual, toric varieties.

• Piene, Ragni (2013). Enriques diagrams and equisingular strata of families of curves. Show summary

  Given a singular point on a curve lying on a smooth surface one associates a weighted sequence of infinitely near points, combinatorially expressed by an Enriques diagram. The singularity defines a fat point on the surface, and the Enriques diagram can be recovered also from the fat point. For a given Enriques diagram and a given surface, the set of fat points with this diagram forms a smooth subscheme of the Hilbert scheme of the surface. The dimension of the subscheme is expressed in terms of the numerical invariants of the diagram, and I will relate the latter to the numerical invariants of the singularity. I will briefly mention how this theory can be applied to study curves in a family with singularities of prescribed topological type. This is joint work with Steven Kleiman.

• Piene, Ragni (2013). Higher order selfdual toric varieties. Show summary

  A lattice point configuration A in Z^n defines a (real or complex) toric embedding in P^N, where N=#A -1. We want to characterize those varieties that are isomorphic to one of its higher order dual varieties, in particular find conditions on the configuration A or its convex lattice polytope P=Conv(A) for this to happen. This is joint work with Alicia Dickenstein, and generalizes previous work by Bourel, Dickenstein, Rittatore in the case of ordinary self dual toric varieties.

• Piene, Ragni (2013). Node polynomials for curves on surfaces. Show summary

  Consider a family of smooth projective surfaces F over Y and a relative divisor D on F. A generalized version of Göttsche's conjecture (proved by Tzeng and Kool–Shende–Thomas in the case of a linear system on a fixed surface) says that the class in Y representing fibres that are r-nodal can be expressed as a Bell polynomial in the relative Chern classes of D and F. We explain the reason for this conjecture and sketch the proof of it in the case r less than or equal to 8.

• Piene, Ragni (2013). Polar varieties revisited. Show summary

  Polar varieties and polar classes have played an important role in the study and classification of projective varieties. In particular they were used to give geometric definitions of Todd classes and Chern classes. Their local counterpart was used in singularity theory. More recently they have been studied and generalized also in the context of real projective and affine geometry, with applications to finding points on real components, to compute Euclidean distance degrees, to caustics by reflection.

• Piene, Ragni (2013). Polytopes, discriminants and toric geometry. Show summary

  This talk will survey recent work by various authors concerning the description of dual and higher order dual varieties of projective toric varieties and the corresponding lattice configurations and polytopes. The interplay between algebraic geometry and combinatorics makes it possible to prove algebro-geometric statements using combinatorial methods, and combinatorial statements using algebro-geometrical methods. An example of the latter is a combinatorial characterization of certain Cayley polytopes.

• Piene, Ragni & Sundlisæter, Tale (2013, 11. april). Profilen: Ragni Piene. [Fagblad].  Teknisk Ukeblad.
• Holden, Helge & Piene, Ragni (2012). Matematikkens gave. Aftenposten (morgenutg. : trykt utg.).  ISSN 0804-3116.
• Piene, Ragni (2012, 21. mars). Abelprisen 2012. [Radio].  Dagsnytt atten - NRK.
• Piene, Ragni (2012). Cayley polytopes and toric geometry. Show summary

  The “Cayley trick” expresses the resultant of two polynomials in one variable as the discriminant of a polynomial in two variables. Generalized and translated into the language of lattice polytopes and toric geometry, the trick consists in building a n-dimensional polytope from k + 1 polytopes of dimension n − k so that the new polytope has no interior lattice points. Such a polytope is called a Cayley polytope. Let P ⊂ R^n be a convex lattice polytope. The codegree of P is an integer between 1 and n + 1 that measures the “hollowness” of P : it is the smallest integer m such that the dilated polytope mP contains interior lattice points. A Cayley polytope as above has codegree at least k + 1. A recent result, due to Dickenstein, Di Rocco, Nill, and Piene, says that the only polytopes of dimension n and codegree at least (n + 3)/2 are Cayley polytopes with k at least (n + 1)/2. The proof relies on the study of the polarized toric variety (X, L) defined by P .

• Piene, Ragni (2012, 30. oktober). Goldbachs formodning. [Radio].  "Det du tror, men ikke kan bevise", Radioselskapet, NRK.
• Piene, Ragni (2012). The Abel Prize – the first 10 years.
• Muntingh, Agnar Georg Peder; Piene, Ragni & Winther, Ragnar (2011). Topics in Polynomial Interpolation Theory. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 1056.
• Piene, Ragni (2011, 24. mars). Abelprisen til banebryter.  Aftenposten.
• Piene, Ragni (2011). Enumerative geometry related to mirror symmetry and SYZ.
• Piene, Ragni (2011). Generating functions and enumerative geometry.
• Piene, Ragni (2011). Generating functions in enumerative geometry. Show summary

  To a sequence of integers $a_1,a_2,a_3,\ldots$ is associated a generating function: the formal power series $f(x)=\sum a_n x^n$. The generating function provides a way of displaying the sequence. For example, if $a_n$ denotes the number of ways one can write the integer $n$ as a sum of positive integers, the generating function is the partition function $p(x)=\Pi _{m\ge 1}(1 - x^m)^{-1}$, and hence one can compute $a_n$ as the coefficient of $x_n$ in the power series expansion of this function. The partition function and its generalization the MacMahon function, as well as the generating functions for the Catalan and the Bell numbers, turn up in recent problems of counting certain geometric objects, such as Gromov--Witten and Donaldson--Thomas invariants. In the talk I will give some examples and some hints at explanations.

• Piene, Ragni (2011). Higher order dual varieties - the toric case. Show summary

  The notion of higher order dual varieties is a natural generalization of the classical notion of dual varieties. They are defined by using higher order osculating spaces instead of tangent spaces. In the case of toric varieties, the degree of the higher order dual varieties can be expressed in combinatorial terms. We shall in particular consider the case of rational normal scrolls, and the case of toric threefolds (joint work with A. Dickenstein and S. Di Rocco).

• Piene, Ragni (2011). Higher order dual varieties: the toric case. Show summary

  Given a projective variety $X\subset \PP^m$, the dual variety $X^{(1)} \subset {\PP^m}^\vee$ is the set of hyperplanes tangent to $X$. More generally, the $k$th dual variety $X^{(k)}$ is the set of hyperplanes tangent to $X$ to order $k$, i.e., containing a $k$th osculating space to $X$. A variety is called $k$-defective if the dimension of $X^{(k)}$ is less than expected. In the case that the embedding $X\subset \PP^m$ is toric, with associated polytope $P$, the degree of $X^{(1)}$ can be expressed in terms of the lattice volumes of the faces of $P$. We show that there is a similar formula for the degree of the $2$-dual of a toric threefold, and we use this expression to show that the only $2$-jet spanned toric embedding that is $2$-defective is $(\PP^3,\mathcal O_{\PP^3}(2))$, i.e., the toric embedding with associated polytope $2\Delta_3$. This is joint work with Alicia Dickenstein and Sandra Di Rocco.

• Piene, Ragni (2011). Matematyka jest wszechobecna. Wiadomości Matematyczne.  47(2), s 221- 223
• Piene, Ragni (2011). The problematic art of counting. Show summary

  Many counting problems, like \begin{itemize} \item[] ``In how many ways can a positive integer $n$ be written as a sum of positive integers?'' \item[] ``Given a polytope $P$, how many lattice points does the dilated polytope $nP$ contain?'' \item[] ``How many lines in a $(n+1)$-dimensional space meet $2n$ general $(n-1) $-planes?'' \end{itemize} are solved by finding a closed form for the corresponding \emph{generating function} $\sum_n N_nq^n$, where the $N_n$ are the sought numbers and $q$ is a variable. In this lecture we shall, in addition to the above questions, also address an old problem from enumerative geometry: \begin{itemize} \item[] ``How many plane curves of degree $d$ have $r$ singularities and pass through $\frac{d(d+3)}{2} -r$ given points in the plane?'' \end{itemize} In this case the generating function is still unknown, but there has recently been substantial progress on the problem and its generalizations.

• D'Andrea, Carlos; Giusti, Marc; Pardo, Luis M. & Piene, Ragni (2010). Effective methods in algebraic geometry 2009: Barcelona. Guest editors' foreword. Journal of symbolic computation.  ISSN 0747-7171.  45(12), s 1251- 1253 . doi: 10.1016/j.jsc.2010.06.007
• Piene, Ragni (2010, 22. august). And who chooses the winners?. [Internett].  http://plus.maths.org/content/and-who-chooses-winners. Show summary

  What's the point of the Fields Medal and other maths prizes? Who decides who gets one? And when will we have the first female medallist? Rachel talks to László Lovász, current president of the International Mathematical Union (IMU), Martin Grötschel, the IMU's secretary, and Ragni Piene, the new chair of the Abel Prize committee, about all this and more.

• Piene, Ragni (2010). Classification des polytopes entiers par fibrations toriques. Show summary

  Un polytope entier $P$ est un polytope dans $\mathbb R^n$ avec sommets dans $\mathbb Z^n$. Un tel polytope peut ou non contenir des points entiers intérieurs. Si non, son double $2P$, ou triple $3P$, ou ... veut en contenir - on appelle le codegre de $P$ le plus petit entier $m$ tel que $mP$ contient des points entiers intérieurs. Le but est de classifier les polytopes tels que m soit grand par rapport à n, ce qu'on achève par des méthodes des variétés dites toriques associées aux polytopes.

• Piene, Ragni (2010). Curve counting and generating functions. Show summary

  Generating functions are a main ingredient in enumerative combinatorics. In enumerative algebraic geometry, however, they were only introduced around 1990, when physicists were able to predict the generating function of the number of rational curves on certain Calabi–Yau threefolds by using the principle of mirror symmetry. Since then, the hunting for generating functions of other curve counting problems has intensified. The talk will explain some successes and some failures in this hunt, with particular focus on singular curves on surfaces.

• Piene, Ragni (2010). Inflection loci of projective varieties. Show summary

  Rational normal curves are the only smooth projective curves that have no inflection points (of any order). In fact this property characterizes Veronese embeddings of projective spaces of any dimension. For a given class of varieties, e.g. toric varieties or scrolls, one can ask to characterize those that are uninflected. In some cases it is possible to compute the degree of the inflection locus, and hence give criteria for it to be empty. The talk will survey some old and new results, and focus on recent joint work with A. Lanteri and R. Mallavibarrena concerning scrolls.

• Piene, Ragni (2010, 18. august). Mathematics ought to be a subject very suited for women.  ICWM Newsletter.
• Piene, Ragni (2010). Some Counting Problems and Their Generating Functions. Show summary

  To a sequence of integers a_1,a_2,a_3, ... is associated with a generating function: the formal power series ƒ(x)=∑a_nX_n. The generating function provides a way of displaying the sequence. For example,if an denotes the number of ways one can write the integer n as a sum of positive integers, the generating function is the partition function p(x)=Π_{m≥1}(1 - x^m)^{-1}, and hence one can compute an as the coefficient of xn in the power series expansion of this function. The partition function and its generalization the MacMahon function, as well as the generating functions for the Catalan and the Bell numbers, turn up in recent problems of counting certain geometric objects (like Gromov-Witten and Donaldson-Thomas invariants). Without going into details, the talk will give some examples and some hints at explanations.

• Piene, Ragni (2009). Counting curves: the hunting of generating functions. Show summary

  Generating functions are a main ingredient in enumerative combinatorics. In enumerative algebraic geometry, however, they were only introduced around 1990, when physicists surprisingly were able to predict the generating function of rational curves on Calabi-Yau threefolds by using the principle of mirror symmetry. Since then, the hunting for generating functions for other curve counting problems has intensified. Ragni Piene will explain some successes and some failures in this hunt, with particular focus on curves on surfaces.

• Piene, Ragni (2009). Functors of infinitely near points on an algebraic surface and the counting of Gromov-Witten invariants.
• Piene, Ragni (2009). Generating functions in enumerative geometry. Show summary

  Abstract. I will explain some problems in enumerative algebraic geometry concerning the counting of curves. In certain cases, solutions exist in the form of explicit generating functions, in other cases there are only conjectures as to the shape of these functions. In particular, I will consider the case of nodal curves on an algebraic surface (joint work with Steve Kleiman).

• Piene, Ragni (2008). Classifying lattice polytopes using toric fibrations.
• Piene, Ragni (2008). Classifying regular lattice polytopes using toric fibrations.
• Piene, Ragni (2008). Classifying regular lattice polytopes via toric fibrations.
• Piene, Ragni (2008). Classifying smooth lattice polytopes via toric fibrations.
• Piene, Ragni (2008). Infinitely near points, Enriques diagrams, and Hilbert schemes.
• Piene, Ragni (2007). Curves and surfaces. Show summary

  This talk is intended as a brief introduction to certain aspects of the theory of algebraic curves and surfaces. To a projective algebraic variety one associates its arithmetic genus, using the constant term of the Hilbert polynomial. Since a complex projective algebraic curve can be viewed as a compact Riemann surface, it also has a topological genus, equal to the number of holes in the Riemann surface. Hirzebruch's Riemann-Roch theorem says that the topological genus is equal to the arithmetic genus. I sketch a proof of this fundamental result, using algebra, geometry, and topology. For algebraic surfaces there is a corresponding result, Noether's formula, expressing the arithmetic genus in terms of topological invariants, which can be proved in a similar way. The last part of the talk discusses the theory of curves on surfaces, in particular the links to classical enumerative geometry and to modern string theory in theoretical physics.

• Piene, Ragni (2007, 07. april). Equal to Nobel.  Frontline, Vol. 24, Issue 07 (India).
• Piene, Ragni (2007). Inflectional loci of scrolls.
• Piene, Ragni (2007). Real monoid surfaces. Show summary

  A monoid surface is a surface of degree d which has a singular point of multiplicity d-1. Any monoid surface admits a rational parameterization, hence is of potential interest in computer aided geometric design. The possible real forms of the singularities on a monoid surface are determined. These results are applied to the classification of quartic monoid surfaces and a study of a stratification of the parameter space of these surfaces. A part of this work is joint with M. Løberg and P.H. Johansen, the other part is due to P.H. Johansen.

• Piene, Ragni (2006). Enumerating singular curves on a surface. Show summary

  Given a family of curves on an algebraic surface and a set of curve singularity types, there is a natural cycle representing curves of the family that have such singularities. The goal is to compute these cycle classes, or even to determine their generating functions. I shall report on joint work with S. Kleiman on this problem.

• Piene, Ragni (2006). Polar and dual varieties of real curves and surfaces. Show summary

  The theory of polar varieties have been used by Bank et al. and Safey El Din et al. to give algorithms for finding a point on each connected component of a smooth, complete intersection real variety. In this talk, based on ongoing joint work with Heidi Mork, we shall consider possibly singular varieties and study the polar varieties and their relation to the dual variety and to the Gauss map. In particular, we shall look at the case of curves and surfaces.

• Piene, Ragni (2006). Zariski clusters, Hilbert schemes, and singular curves on a surface. Show summary

  This is a report on joint work with Steven Kleiman. We consider a family of curves on a surface, parameterized by some variety Y. Then Y has a stratification, where the points of a stratum correspond to curves of a given singularity type. The singularity type is encoded by an Enriques diagram. This leads to the study of certain subschemes of the Hilbert scheme of the surface, whose points correspond to Zariski clusters with a given Enriques diagram. We apply this to the question of enumerating curves having given singularities. In particular, we look for the generating function for the number of curves with a fixed number of nodes, and we show how the Bell polynomials naturally appear in this problem.

• Piene, Ragni (2005). Bell polynomials and enumerative geometry.
• Piene, Ragni (2005). Counting curves on a surface.
• Piene, Ragni (2005). Generating functions and enumerative geometry.
• Piene, Ragni (2005). The curve counting problem.
• Piene, Ragni (2005). The curve counting problem.
• Piene, Ragni (2005). The curve counting problem - or: when algebraic geometry met string theory.
• Piene, Ragni (2004). Cuspidal plane curves.
• Piene, Ragni (2004). Enriques diagrams and equisingular strata of families of curves.
• Piene, Ragni (2004). Enriques diagrams and the equisingular stratification of famlies of curves.
• Piene, Ragni (2004). Enumerating and characterizing real and complex singularities of curves and surfaces.
• Piene, Ragni (2004). The equisingular stratification of families of curves on surfaces.
• Piene, Ragni (2004). The equisingular stratification of families of curves on surfaces.
• Piene, Ragni (2003). On the classification of real algebraic curves and surfaces.
• Piene, Ragni (2003). On the use of classical algebraic geometry in computer aided design.
• Piene, Ragni (2003, 27. september). The numbers game.  New Scientist.
• Piene, Ragni (2002). Counting curves on Abelian surfaces.
• Piene, Ragni (2002). Hilbert schemes and singular curves on a surface. Show summary

  This is a report on joint work with S. Kleiman. In a series of papers we have developed methods to enumerate singular curves on smooth surfaces. The goal is to express the enumerating cycle as a polynomial in classes obtained from the Chern classes of the family of curves. To a singularity of a curve on a smooth surface $S$ we can associate a cluster --- a finite subscheme of $S$ of length equal to the {\sl degree} of the Enriques diagram of the singularity. Let $\pi:F\to Y$ be a family of surfaces and $D$ a relative, effective divisor. Let ${\bf D}$ be a set of Enriques diagrams, set $d:={\rm deg}\,{\bf D}$, and let $H({\bf D})\subseteq {\rm Hilb}^d_{F/Y}$ denote the subscheme consisting of clusters with diagram ${\bf D}$. The cycle enumerating curves in the family $D$ that have singularities of type ${\bf D}$ is defined to be the pushdown to $Y$ of the class $Z({\bf D})$ corresponding to $H({\bf D})\cap {\rm Hilb}^d_{D/Y}$ in ${\rm Hilb}^d_{F/Y}$. The subschemes $H({\bf D})$ are smooth, irreducible and of dimension equal to ${\rm dim}\,{\bf D}$. Unfortunately, not enough is known about the cohomology ring of the Hilbert schemes in order to compute $Z({\bf D})$ directly, so we use a recursive procedure instead.

• Piene, Ragni (2001, 21. juni). Den fantastiske matematikken.  Dag og Tid.
• Piene, Ragni (2001). Familles des courbes sur des surfaces: le problème d'énumeration des courbes équisingulières. Show summary

  On étudie des familles des courbes sur une famille de surfaces lisses, et on s'interesse aux problèmes énumeratifs des courbes équisingulières. Le type de singularité d'une courbe sur une surface lisse est determiné par son diagramme d'Enriques, ou bien par un idéal complet de la surface, donc un élément dans le schéma d'Hilbert. On montre que la strate des points, dans le schéma d'Hilbert, correspondants aux courbes équisingulières (d'un type donné), est lisse et irréductible. Pour énumerer les courbes d'un type donné, on cherche une expression pour le cycle fondamentale de cette strate intersectée avec un schéma d'Hilbert de la famille des courbes; on montre comment on peut réaliser ce cycle dans l'espace de paramètres de la famille des courbes. En utilisant une récursion par éclatements, on peut obtenir des formules dans des cas particuliers.

• Piene, Ragni (2001, 17. april). Matematikk = poesi.  Dagbladet.

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