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TENSOR RANK AND THE ILLPOSEDNESS OF THE BEST LOWRANK APPROXIMATION PROBLEM
"... There has been continued interest in seeking a theorem describing optimal lowrank approximations to tensors of order 3 or higher, that parallels the Eckart–Young theorem for matrices. In this paper, we argue that the naive approach to this problem is doomed to failure because, unlike matrices, te ..."
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Cited by 75 (10 self)
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There has been continued interest in seeking a theorem describing optimal lowrank approximations to tensors of order 3 or higher, that parallels the Eckart–Young theorem for matrices. In this paper, we argue that the naive approach to this problem is doomed to failure because, unlike matrices, tensors of order 3 or higher can fail to have best rankr approximations. The phenomenon is much more widespread than one might suspect: examples of this failure can be constructed over a wide range of dimensions, orders and ranks, regardless of the choice of norm (or even Brègman divergence). Moreover, we show that in many instances these counterexamples have positive volume: they cannot be regarded as isolated phenomena. In one extreme case, we exhibit a tensor space in which no rank3 tensor has an optimal rank2 approximation. The notable exceptions to this misbehavior are rank1 tensors and order2 tensors (i.e. matrices). In a more positive spirit, we propose a natural way of overcoming the illposedness of the lowrank approximation problem, by using weak solutions when true solutions do not exist. For this to work, it is necessary to characterize the set of weak solutions, and we do this in the case of rank 2, order 3 (in arbitrary dimensions). In our work we emphasize the importance of closely studying concrete lowdimensional examples as a first step towards more general results. To this end, we present a detailed analysis of equivalence classes of 2 × 2 × 2 tensors, and we develop methods for extending results upwards to higher orders and dimensions. Finally, we link our work to existing studies of tensors from an algebraic geometric point of view. The rank of a tensor can in theory be given a semialgebraic description; in other words, can be determined by a system of polynomial inequalities. We study some of these polynomials in cases of interest to us; in particular we make extensive use of the hyperdeterminant ∆ on R 2×2×2.
Symmetric tensors and symmetric tensor rank
 Scientific Computing and Computational Mathematics (SCCM
, 2006
"... Abstract. A symmetric tensor is a higher order generalization of a symmetric matrix. In this paper, we study various properties of symmetric tensors in relation to a decomposition into a symmetric sum of outer product of vectors. A rank1 orderk tensor is the outer product of k nonzero vectors. An ..."
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Cited by 46 (19 self)
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Abstract. A symmetric tensor is a higher order generalization of a symmetric matrix. In this paper, we study various properties of symmetric tensors in relation to a decomposition into a symmetric sum of outer product of vectors. A rank1 orderk tensor is the outer product of k nonzero vectors. Any symmetric tensor can be decomposed into a linear combination of rank1 tensors, each of them being symmetric or not. The rank of a symmetric tensor is the minimal number of rank1 tensors that is necessary to reconstruct it. The symmetric rank is obtained when the constituting rank1 tensors are imposed to be themselves symmetric. It is shown that rank and symmetric rank are equal in a number of cases, and that they always exist in an algebraically closed field. We will discuss the notion of the generic symmetric rank, which, due to the work of Alexander and Hirschowitz, is now known for any values of dimension and order. We will also show that the set of symmetric tensors of symmetric rank at most r is not closed, unless r = 1. Key words. Tensors, multiway arrays, outer product decomposition, symmetric outer product decomposition, candecomp, parafac, tensor rank, symmetric rank, symmetric tensor rank, generic symmetric rank, maximal symmetric rank, quantics AMS subject classifications. 15A03, 15A21, 15A72, 15A69, 15A18 1. Introduction. We
Enumerative Geometry For The Real Grassmannian Of Lines In Projective Space
 Duke Math. J
, 1996
"... Given Schubert conditions on lines in projective space which generically determine a finite number of lines, we show there exist general real conditions determining the expected number of real lines. This extends the classical Schubert calculus of enumerative geometry for the Grassmann variety of ..."
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Cited by 30 (16 self)
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Given Schubert conditions on lines in projective space which generically determine a finite number of lines, we show there exist general real conditions determining the expected number of real lines. This extends the classical Schubert calculus of enumerative geometry for the Grassmann variety of lines in projective space from the complex realm to the real. Our main tool is an explicit geometric description of rational equivalences, which also constitutes a novel determination of the Chow rings of these Grassmann varieties of lines.
On the passage from local to global in number theory
 Bull. Amer. Math. Soc. (N.S
, 1993
"... Would a reader be able to predict the branch of mathematics that is the subject of this article if its title had not included the phrase “in Number Theory”? The distinction “local ” versus “global”, with various connotations, has found a home in almost every part of mathematics, local problems being ..."
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Cited by 21 (0 self)
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Would a reader be able to predict the branch of mathematics that is the subject of this article if its title had not included the phrase “in Number Theory”? The distinction “local ” versus “global”, with various connotations, has found a home in almost every part of mathematics, local problems being often a steppingstone to
Linear Systems of Plane Curves
 Notices of the AMS
, 1999
"... Interpolation with polynomials is a subject that has occupied mathematicians ’ minds for millenia. The general problem can be informally phrased as: Given a set of points {(xi,yi)} in the plane, find a ..."
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Cited by 21 (1 self)
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Interpolation with polynomials is a subject that has occupied mathematicians ’ minds for millenia. The general problem can be informally phrased as: Given a set of points {(xi,yi)} in the plane, find a
The Algebraic Degree of Semidefinite Programming
 LEIBNITZ UNIVERSITÄT HANNOVER, WELFENGARTEN 1, D30167 HANNOVER EMAIL ADDRESS: BOTHMER@MATH.UNIHANNOVER.DE URL: HTTP://WWW.IAG.UNIHANNOVER.DE/~BOTHMER/ MATEMATISK INSTITUTT, UNIVERSITETET I OSLO, PO BOX 1053, BLINDERN, NO0316
, 2008
"... Given a generic semidefinite program, specified by matrices with rational entries, each coordinate of its optimal solution is an algebraic number. We study the degree of the minimal polynomials of these algebraic numbers. Geometrically, this degree counts the critical points attained by a linear fu ..."
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Cited by 18 (8 self)
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Given a generic semidefinite program, specified by matrices with rational entries, each coordinate of its optimal solution is an algebraic number. We study the degree of the minimal polynomials of these algebraic numbers. Geometrically, this degree counts the critical points attained by a linear functional on a fixed rank locus in a linear space of symmetric matrices. We determine this degree using methods from complex algebraic geometry, such as projective duality, determinantal varieties, and their Chern classes.
pelementary subgroups of the Cremona group
 J. of Algebra
"... Let k be an algebraically closed field. The Cremona group Crk is the group of birational transformations of P2 k, or equivalently the group of kautomorphisms of the field k(x,y). There is an extensive classical literature about this group, in particular about its finite subgroups – see the introduc ..."
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Cited by 16 (2 self)
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Let k be an algebraically closed field. The Cremona group Crk is the group of birational transformations of P2 k, or equivalently the group of kautomorphisms of the field k(x,y). There is an extensive classical literature about this group, in particular about its finite subgroups – see the introduction of [dF] for a list of
Geometry and the complexity of matrix multiplication
, 2007
"... Abstract. We survey results in algebraic complexity theory, focusing on matrix multiplication. Our goals are (i) to show how open questions in algebraic complexity theory are naturally posed as questions in geometry and representation theory, (ii) to motivate researchers to work on these questions, ..."
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Cited by 15 (1 self)
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Abstract. We survey results in algebraic complexity theory, focusing on matrix multiplication. Our goals are (i) to show how open questions in algebraic complexity theory are naturally posed as questions in geometry and representation theory, (ii) to motivate researchers to work on these questions, and (iii) to point out relations with more general problems in geometry. The key geometric objects for our study are the secant varieties of Segre varieties. We explain how these varieties are also useful for algebraic statistics, the study of phylogenetic invariants, and quantum computing.
Dynamic Pole Assignment and Schubert Calculus
 SIAM J. Control and Optim
, 1996
"... . The output feedback pole assignment problem is a classical problem in linear systems theory. In this paper we calculate the number of complex dynamic compensators of order q assigning a given set of poles for a qnondegenerate minput, poutput system of McMillan degree n = q(m+p \Gamma 1) + mp. A ..."
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Cited by 14 (5 self)
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. The output feedback pole assignment problem is a classical problem in linear systems theory. In this paper we calculate the number of complex dynamic compensators of order q assigning a given set of poles for a qnondegenerate minput, poutput system of McMillan degree n = q(m+p \Gamma 1) + mp. As a corollary it follows that when this number is odd, the generic system can be arbitrarily pole assigned by output feedback with a real dynamic compensator of order at most q if and only if q(m + p \Gamma 1) + mp n. Key words. output feedback pole assignment, dynamic compensator, holomorphic curves in Grassmannian, degree of variety AMS subject classifications. 93B55, 93B27, 14M15 1. Introduction. The output feedback pole assignment of linear systems with static or dynamic compensators is a classical problem in control theory and many theoretical and numerical research papers have been devoted to this problem. Although the systems involved are linear, the problem is in fact not linear...
Sextic double solids
, 2004
"... Abstract. We prove nonrationality and birational superrigidity of a Qfactorial double cover X of P 3 ramified along a sextic surface with at most simple double points. We also show that the condition #Sing(X)  ≤ 14 implies Qfactoriality of X. In particular, every double cover of P 3 with at m ..."
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Cited by 12 (5 self)
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Abstract. We prove nonrationality and birational superrigidity of a Qfactorial double cover X of P 3 ramified along a sextic surface with at most simple double points. We also show that the condition #Sing(X)  ≤ 14 implies Qfactoriality of X. In particular, every double cover of P 3 with at most 14 simple double points is nonrational and not birationally isomorphic to a conic bundle. All the birational transformations of X into elliptic fibrations and into Fano 3folds with canonical singularities are classified. We consider some relevant problems over fields of finite characteristic. When X is defined over a number field F we prove that the set of rational points on the 3fold X is potentially dense if Sing(X) ̸ = ∅.