Shmuel Friedland
We study tensors in $\C^{m\times n\times l}$ whose border rank is $l$. We characterize the tensors in $\C^{3\times 3\times 4}$ and in $\C^{4\times 4\times 4}$ of border rank 4 at most.
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Fix integers $m \ge 2$, $s\ge 5$ and $d\ge 2s+2$. Here we describe the possible symmetric tensor ranks $\le 2d+s-7$ of all symmetric tensors (or homogeneous degree $d$ polynomials) in $m+1$ variables with border rank $s$.
In this work we give the classification of real ternary cubic forms with respect to rank and border rank up to SL(3)-action and we examine the differences with the complex case.
We investigate an extension of a lower bound on the Waring (cactus) rank of homogeneous forms due to Ranestad and Schreyer. We show that for particular classes of homogeneous forms, for which a generalization of this method applies, the lower bound extends to the level of border (cactus) rank. The approach is based on recent results on tensor asymptotic rank.
We consider higher secant varieties to Veronese varieties. Most points on the r-th secant variety are represented by a finite scheme of length r contained in the Veronese variety --- in fact, for generic point, it is just a union of r distinct points. A modern way to phrase it is: the smoothable rank is equal to the border rank for most polynomials. This property is very useful for studying secant varieties, especially, whenever the smoothable rank is equal to the border rank for all points of the secant variety in question. In this note we investigate those special points for which the smooth…
Motivated by questions arising in signal processing, computational complexity, and other areas, we study the ranks and border ranks of symmetric tensors using geometric methods. We provide improved lower bounds for the rank of a symmetric tensor (i.e., a homogeneous polynomial) obtained by considering the singularities of the hypersurface defined by the polynomial. We obtain normal forms for polynomials of border rank up to five, and compute or bound the ranks of several classes of polynomials, including monomials, the determinant, and the permanent.
We introduce an elementary method to study the border rank of polynomials and tensors, analogous to the apolarity lemma. This can be used to describe the border rank of all cases uniformly, including those very special ones that resisted a systematic approach. We also define a border rank version of the variety of sums of powers and analyse its usefulness in studying tensors and polynomials with large symmetries. In particular, it can be applied to provide lower bounds for the border rank of some very interesting tensors, such as the matrix multiplication tensor. We work in a general setting, …
Wild forms are homogeneous polynomials whose smoothable rank is strictly larger than their border rank. The discrepancy between these two ranks is caused by the difference between the limit of spans of a family of zero-dimensional schemes and the span of their flat limit. For concise forms of minimal border rank, we show that the condition of vanishing Hessian is equivalent to being wild. This is proven by making a detour through structure tensors of smoothable and Gorenstein algebras. The equivalence fails in the non-minimal border rank regime. We exhibit an infinite series of minimal border …
Border rank bounds for $GL(V)$-invariant tensors arising from matrices of constant rank
本文研究由常秩矩阵空间产生的 $GL(V)$-不变张量，利用 Young flattenings 证明新的边界秩下界，并确定三个张量的边界秩。
We introduce various notions of rank for a symmetric tensor, namely: rank, border rank, catalecticant rank, generalized rank, scheme length, border scheme length, extension rank and smoothable rank. We analyze the stratification induced by these ranks. The mutual relations between these stratifications, allow us to describe the hierarchy among all the ranks. We show that strict inequalities are possible between rank, border rank, extension rank and catalecticant rank. Moreover we show that scheme length, generalized rank and extension rank coincide.
The structure tensor of $\mathfrak{sl}_n$, denoted $T_{\mathfrak{sl}_n}$, is the tensor arising from the Lie bracket bilinear operation on the set of traceless $n \times n$ matrices over $\mathbb{C}$. This tensor is intimately related to the well studied matrix multiplication tensor. Studying the structure tensor of $\mathfrak{sl}_n$ may provide further insight into the complexity of matrix multiplication and the "hay in a haystack" problem of finding explicit sequences tensors with high rank or border rank. We aim to find new bounds on the rank and border rank of this structure tensor in the …
We prove a lower bound on the dimension of the set of maximal border subrank tensors. This is the first such bound of its type.
Let $σ_b(X_{m,d}(\mathbb {C}))(\mathbb {R})$, $b(m+1) < \binom{m+d}{m}$, denote the set of all degree $d$ real homogeneous polynomials in $m+1$ variables (i.e. real symmetric tensors of format $(m+1)\times ... \times (m+1)$, $d$ times) which have border rank $b$ over $\mathbb {C}$. It has a partition into manifolds of real dimension $\le b(m+1)-1$ in which the real rank is constant. A typical rank of $σ_b(X_{m,d}(\mathbb {C}))(\mathbb {R})$ is a rank associated to an open part of dimension $b(m+1)-1$. Here we classify all typical ranks when $b\le 7$ and $d, m$ are not too small. For a larger s…
We give the stratification by the symmetric tensor rank of all degree $d \ge 9$ homogeneous polynomials with border rank $5$ and which depend essentially on at least 5 variables, extending previous works (A. Bernardi, A. Gimigliano, M. Idà, E. Ballico) on lower border ranks. For the polynomials which depend on at least 5 variables only 5 ranks are possible: $5$, $d+3$, $2d+1$, $3d-1$, $4d-3$, but each of the ranks $3d-1$ and $2d+1$ is achieved in two geometrically different situations. These ranks are uniquely determined by a certain degree 5 zero-dimensional scheme $A$ associated to the polyn…
The $X$-rank of a point $p$ in projective space is the minimal number of points of an algebraic variety $X$ whose linear span contains $p$. This notion is naturally submultiplicative under tensor product. We study geometric conditions that guarantee strict submultiplicativity. We prove that in the case of points of rank two, strict submultiplicativity is entirely characterized in terms of the trisecant lines to the variety. Moreover, we focus on the case of curves: we prove that for curves embedded in an even-dimensional projective space, there are always points for which strict submultiplicat…
For odd n, I write down tensors in C^n\otimes C^n\otimes C^n of border rank 2n-1, showing the non-triviality of the Young-flattening equations of Landsberg-Ottaviani. I also study the border rank of the tensors of Alexeev et. al., showing the tensors their tensors T_{2^k}, despite having rank equal to 2^{k+1}-1, have border rank equal to 2^k, the minimum of any concise tensor. I also study the equations of Griesser.
Border rank of powers of ternary quadratic forms
本文主要研究三元二次型任意次幂的边界秩，通过确定配极理想并利用边界配极技术，证明任意次幂 q_3^s 的边界秩等于其中间卡塔莱克坦矩阵的秩 (s+1)(s+2)/2。
Border rank=rank for Kruskal tensors and a Kruskal's theorem for skew decompositions
该研究证明Kruskal张量的边界秩等于秩，给出交替张量的Kruskal条件以证明斜秩分解唯一性，还证明交替Kruskal张量的边界斜秩等于斜秩并给出最小斜秩分解算法。
It has recently been shown that the tensor rank can be strictly submultiplicative under the tensor product, where the tensor product of two tensors is a tensor whose order is the sum of the orders of the two factors. The necessary upper bounds were obtained with help of border rank. It was left open whether border rank itself can be strictly submultiplicative. We answer this question in the affirmative. In order to do so, we construct lines in projective space along which the border rank drops multiple times and use this result in conjunction with a previous construction for a tensor rank drop…
Some concerns on the border rank of Kronecker products of the Coppersmith-Winograd tensor
本文详细证明了小 Coppersmith-Winograd 张量的 Kronecker 平方的边界秩为 (q+2)^2，并指出该方法有望推广到 Kronecker 立方及更高次幂，支持猜想值 (q+2)^m。
Let M_n denote the matrix multiplication tensor for nxn matrices. We use the border substitution method combined with Koszul flattenings to prove the border rank lower bound of 2n^2-log(n)-1 for M_n.
We establish basic information about border rank algorithms for the matrix multiplication tensor and other tensors with symmetry. We prove that border rank algorithms for tensors with symmetry (such as matrix multiplication and the determinant polynomial) come in families that include representatives with normal forms. These normal forms will be useful both to develop new efficient algorithms and to prove lower complexity bounds. We derive a border rank version of the substitution method used in proving lower bounds for tensor rank. We use this border-substitution method and a normal form to i…
We write down an explicit sequence of tensors in $C^m\otimes C^m\otimes C^m$, for all $m$ sufficiently large, having border rank at least $2.02m$, overcoming a longstanding barrier. We obtain our lower bounds via the border substitution method.
In the first part of this paper we study a best approximation of a vector in Euclidean space R^n with respect to a closed semi-algebraic set C and a given semi-algebraic norm. Assuming that the given norm and its dual norm are differentiable we show that a best approximation is unique outside a hypersurface. We then study the case where C is an irreducible variety and the approximation is with respect to the Euclidean norm. We show that for a general point in x in R^n the number of critical points of the distance function of x to C is bounded above by a degree of a related dominant map. If C i…
Symmetrization maps and minimal border rank Comon's conjecture
本文针对边界科蒙猜想，利用边界零化性和幂和的边界簇作为技术工具，证明了(ℂⁿ)⊗ᵈ中边界秩为n的大量简洁张量类的该猜想成立，涵盖所有驯服张量及n≤d+1时的所有张量。
We present new lower bounds for the symmetric border rank of the n x n determinant for all n. Further lower bounds are given for the 3 x 3 permanent.
Border apolarity and varieties of sums of powers
本文研究幂和簇的边界簇（$\underline{\mathrm{VSP}}$'s），比较其在 Hilbert 概形中的轨迹，引入边界可辨识性概念并提供判据，最后在张量和齐次多项式背景下确定若干实例的 $\underline{\mathrm{VSP}}$'s。
Whereas matrix rank is additive under direct sum, in 1981 Schönhage showed that one of its generalizations to the tensor setting, tensor border rank, can be strictly subadditive for tensors of order three. Whether border rank is additive for higher order tensors has remained open. In this work, we settle this problem by providing analogs of Schönhage's construction for tensors of order four and higher. Schönhage's work was motivated by the study of the computational complexity of matrix multiplication; we discuss implications of our results for the asymptotic rank of higher order generalizatio…
Polynomials of minimal border rank
利用Gorenstein代数的迭代乘法张量与极小可光滑秩齐次多项式之间的对应，分类了至多7个变量中足够高次数的极小边界秩多项式。
Border rank lower bounds beyond weak border apolarity
本文首次展示超越弱边界对偶性的边界秩下界示例，证明某些张量的理想列表虽非空但无一满足全部条件，涵盖矩阵乘法相关张量及群论相关案例。
The border Waring rank of $x_1\cdots x_n$ is $2^{n-1}$
本文证明特征零域上单项式 $x_1\cdots x_n$ 的边界 Waring 秩为 $2^{n-1}$，表明极化恒等式为最优分解，下界证明借助非对称的行列式张量的高阶 Koszul 展平。
We determine the border ranks of tensors that could potentially advance the known upper bound for the exponent $ω$ of matrix multiplication. The Kronecker square of the small $q=2$ Coppersmith-Winograd tensor equals the $3\times 3$ permanent, and could potentially be used to show $ω=2$. We prove the negative result for complexity theory that its border rank is $16$, resolving a longstanding problem. Regarding its $q=4$ skew cousin in $ C^5\otimes C^5\otimes C^5$, which could potentially be used to prove $ω\leq 2.11$, we show the border rank of its Kronecker square is at most $42$, a remarkable…
We prove that the border rank of the Kronecker square of the little Coppersmith-Winograd tensor $T_{cw,q}$ is the square of its border rank for $q > 2$ and that the border rank of its Kronecker cube is the cube of its border rank for $q > 4$. This answers questions raised implicitly in [Coppersmith-Winograd, 1990] and explicitly in [Bläser, 2013] and rules out the possibility of proving new upper bounds on the exponent of matrix multiplication using the square or cube of a little Coppersmith-Winograd tensor in this range. In the positive direction, we enlarge the list of explicit tensors poten…
We study cubic surfaces as symmetric tensors of format $4 \times 4 \times 4$. We consider the non-symmetric tensor rank and the symmetric Waring rank of cubic surfaces, and show that the two notions coincide over the complex numbers. The corresponding algebraic problem concerns border ranks. We show that the non-symmetric border rank coincides with the symmetric border rank for cubic surfaces. As part of our analysis, we obtain minimal ideal generators for the symmetric analogue to the secant variety from the salmon conjecture. We also give a test for symmetric rank given by the non-vanishing …
An important building block in all current asymptotically fast algorithms for matrix multiplication are tensors with low border rank, that is, tensors whose border rank is equal or very close to their size. To find new asymptotically fast algorithms for matrix multiplication, it seems to be important to understand those tensors whose border rank is as small as possible, so called tensors of minimal border rank. We investigate the connection between degenerations of associative algebras and degenerations of their structure tensors in the sense of Strassen. It allows us to describe an open subse…
The border rank of the $4 \times 4$ determinant tensor is twelve
该研究通过特定方法证明复数域上4×4行列式张量的边界秩至少为12，结合已知上界确定其边界秩为12，明确了该张量的边界秩精确值。
The border rank of the matrix multiplication operator for n by n matrices is a standard measure of its complexity. Using techniques from algebraic geometry and representation theory, we show the border rank is at least 2n^2-n. Our bounds are better than the previous lower bound (due to Lickteig in 1985) of 3/2 n^2+ n/2 -1 for all n>2. The bounds are obtained by finding new equations that bilinear maps of small border rank must satisfy, i.e., new equations for secant varieties of triple Segre products, that matrix multiplication fails to satisfy.
We present new methods for determining polynomials in the ideal of the variety of bilinear maps of border rank at most r. We apply these methods to several cases including the case r = 6 in the space of bilinear maps C^4 x C^4 -> C^4. This space of bilinear maps includes the matrix multiplication operator M_2 for two by two matrices. We show these newly obtained polynomials do not vanish on the matrix multiplication operator M_2, which gives a new proof that the border rank of the multiplication of 2 x 2 matrices is seven. Other examples are considered along with an explanation of how to imple…
Hankel tensors are generalizations of Hankel matrices. This article studies the relations among various ranks of Hankel tensors. We give an algorithm that can compute the Vandermonde ranks and decompositions for all Hankel tensors. For a generic $n$-dimensional Hankel tensor of even order or order three, we prove that the the cp rank, symmetric rank, border rank, symmetric border rank, and Vandermonde rank all coincide with each other. In particular, this implies that the Comon's conjecture is true for generic Hankel tensors when the order is even or three. Some open questions are also posed.
On the geometry of border rank algorithms for n x 2 by 2 x 2 matrix multiplication
研究n×2与2×2矩阵乘法的边界秩算法几何特性，提出新的分析方法和优化策略。
Fixed-parameter debordering of Waring rank
本研究首次获得多项式Waring秩的上界，该上界关于边界Waring秩指数增长、关于次数线性增长，解决了去边界化问题，并改进了常数边界Waring秩情形下的结果。
Classification and degenerations of small minimal border rank tensors via modules
本文通过模方法给出了 $m \leq 5$ 时 $1_*$-generic 极小边界秩张量的自包含分类，结合已有结果完成了该范围内所有极小边界秩张量的分类，并完整描述了张量间的退化情况。
Border subrank of higher order tensors and algebras
研究高阶结构张量的边界子秩，确定k重矩阵乘法和上三角矩阵乘法的紧界，分析多项式代数、空代数及二次的极化代数的高阶结构张量边界子秩，并证明代数结构张量退化从高阶到低阶的传播。
Concise tensors of minimal border rank
借助111-方程与交换矩阵簇结果，引入简洁张量的111-代数不变量，确定了m=5,6时极小边界秩简洁张量的定义方程并完成分类。
Border Ranks of Positive and Invariant Tensor Decompositions: Applications to Correlations
本研究证明多体正定与不变张量分解存在秩与边界秩的差距，并利用该差距证明相关概率分布集合非闭集，导致有限时间内无法测试其成员资格，揭示了秩的不稳定性与双体系统的独特性。
Border Ranks of Monomials
本文研究Young展平方法对单项式边界秩的下界极限，给出最优形状，覆盖次数≤6、3变量及变量乘积幂，并指出高次时无法匹配猜想紧上界。
Cactus barriers
本文通过仙人掌种群和代数几何方法解释了确定性方法在张量边界秩界限定制中的理论障碍。
The border rank of the multiplication of two by two matrices is seven
本文证明，近似计算2x2矩阵乘法至少需要七次乘法，即矩阵乘法算子不在三个P^3的Segre乘积的第六割线簇中。
Border rank lower bounds for families of GL(V)-invariant tensors
本文针对U⊗S_λV⊗S_μV中GL(V)不变张量族（U为V、Sym²V或∧²V）给出边界秩非平凡下界，解决Wu猜想并推广技术，扩展非最小边界秩例子数量。
Unrestrictions and concise secant varieties
本文基于简洁secant族，探讨了张量理论中border rank与cactus rank的联系，以及与Segre嵌入、Salmon猜想等的关联，提出简洁border apolarity和Sums of Powers varieties的刻画。
Uniformity for limits of tensors
本文研究多线性代数中各类秩的边界秩问题，证明去边界化定理并揭示实现边界秩的极限复杂度不随维数增长，基于GL-簇理论建立统一框架。
We give an upper bound for the rank of the border rank 3 partially symmetric tensors. In the special case of border rank 3 tensors $T\in V_1\otimes \cdots \otimes V_k$ (Segre case) we can show that all ranks among 3 and $k-1$ arise and if $\dim V_i \geq 3$ for all $i$'s, then also all the ranks between $k$ and $2k-1$ arise.
We study three variations of the Waring problem for polynomials, concerning the Waring rank, the border rank and the cactus rank of a form and we show how the Lefschetz properties of the associated algebra affect them. The main tool is the theory of mixed Hessian matrix. We construct new families of wild forms, that is, forms whose cactus rank, of schematic nature, is bigger then the border rank, defined geometrically.
These lecture notes are intended as an introduction to several notions of tensor rank and their connections to the asymptotic complexity of matrix multiplication. The latter is studied with the exponent of matrix multiplication, which will be expressed in terms of tensor (border) rank, (border) symmetric rank and the asymptotic rank of certain tensors. We introduce the multilinear rank of a tensor as well, deal with the concept of tensor equivalence and study prehomogeneous vector spaces with the castling transform. Moreover, we treat Apolarity Theory and use it to determine the symmetric rank…
We address the problem of the additivity of the tensor rank. That is for two independent tensors we study if the rank of their direct sum is equal to the sum of their individual ranks. A positive answer to this problem was previously known as Strassen's conjecture until recent counterexamples were proposed by Shitov. The latter are not very explicit, and they are only known to exist asymptotically for very large tensor spaces. In this article we prove that for some small three-way tensors the additivity holds. For instance, if the rank of one of the tensors is at most 6, then the additivity ho…
Topology of tensor ranks
本文研究了由张量秩、边界秩、多线性秩及其对称形式定义的张量集合的路径连通性和同调群，探讨了复数和实数域上的连通性及同调群特性。
Explicit tensors beyond the linear flattening barrier
本文构造了边界秩至少为3n-o(n)的显式张量，并证明线性展平方法无法超越2n-1的下界，从而提供了超越线性展平障碍的显式示例。
Asymptotic rank bounds: a numerical census
本文通过数值方法计算张量的改进渐近秩界，验证了Strassen猜想并揭示了现有数值方法的限制。
It has been shown that a best rank-R approximation of an order-k tensor may not exist when R>1 and k>2. This poses a serious problem to data analysts using tensor decompositions. It has been observed numerically that, generally, this issue cannot be solved by consecutively computing and subtracting best rank-1 approximations. The reason for this is that subtracting a best rank-1 approximation generally does not decrease tensor rank. In this paper, we provide a mathematical treatment of this property for real-valued 2x2x2 tensors, with symmetric tensors as a special case. Regardless of the symm…
We show that the product rank of the $3\times 3$ determinant is $5$, and the product rank of the $3\times 3$ permanent is $4$. As a corollary, we obtain that the tensor ranks of the $3 \times 3$ determinant and permanent are $5$ and $4$, respectively. We show moreover that the border product rank of the $n \times n$ permanent is larger than $n$ for any $n\geq 3$.
We lower bound the rank of a tensor by a linear combination of the ranks of three of its unfoldings, using Sylvester's rank inequality. In a similar way, we lower bound the symmetric rank by a linear combination of the symmetric ranks of three unfoldings. Lower bounds on the rank and symmetric rank of tensors are important for finding counterexamples to Comon's conjecture. A real counterexample to Comon's conjecture is a tensor whose real rank and real symmetric rank differ. Previously, only one real counterexample was known. We divide the construction into three steps. The first step involves…
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