1,722 research outputs found
Asymptotic tensor rank of graph tensors: beyond matrix multiplication
We present an upper bound on the exponent of the asymptotic behaviour of the
tensor rank of a family of tensors defined by the complete graph on
vertices. For , we show that the exponent per edge is at most 0.77,
outperforming the best known upper bound on the exponent per edge for matrix
multiplication (), which is approximately 0.79. We raise the question
whether for some the exponent per edge can be below , i.e. can
outperform matrix multiplication even if the matrix multiplication exponent
equals 2. In order to obtain our results, we generalise to higher order tensors
a result by Strassen on the asymptotic subrank of tight tensors and a result by
Coppersmith and Winograd on the asymptotic rank of matrix multiplication. Our
results have applications in entanglement theory and communication complexity
The asymptotic spectrum of graphs and the Shannon capacity
We introduce the asymptotic spectrum of graphs and apply the theory of
asymptotic spectra of Strassen (J. Reine Angew. Math. 1988) to obtain a new
dual characterisation of the Shannon capacity of graphs. Elements in the
asymptotic spectrum of graphs include the Lov\'asz theta number, the fractional
clique cover number, the complement of the fractional orthogonal rank and the
fractional Haemers bounds
Faster Algorithms for Rectangular Matrix Multiplication
Let {\alpha} be the maximal value such that the product of an n x n^{\alpha}
matrix by an n^{\alpha} x n matrix can be computed with n^{2+o(1)} arithmetic
operations. In this paper we show that \alpha>0.30298, which improves the
previous record \alpha>0.29462 by Coppersmith (Journal of Complexity, 1997).
More generally, we construct a new algorithm for multiplying an n x n^k matrix
by an n^k x n matrix, for any value k\neq 1. The complexity of this algorithm
is better than all known algorithms for rectangular matrix multiplication. In
the case of square matrix multiplication (i.e., for k=1), we recover exactly
the complexity of the algorithm by Coppersmith and Winograd (Journal of
Symbolic Computation, 1990).
These new upper bounds can be used to improve the time complexity of several
known algorithms that rely on rectangular matrix multiplication. For example,
we directly obtain a O(n^{2.5302})-time algorithm for the all-pairs shortest
paths problem over directed graphs with small integer weights, improving over
the O(n^{2.575})-time algorithm by Zwick (JACM 2002), and also improve the time
complexity of sparse square matrix multiplication.Comment: 37 pages; v2: some additions in the acknowledgment
Cryptography from tensor problems
We describe a new proposal for a trap-door one-way function. The new proposal belongs to the "multivariate quadratic" family but the trap-door is different from existing methods, and is simpler
Barriers for fast matrix multiplication from irreversibility
Determining the asymptotic algebraic complexity of matrix multiplication,
succinctly represented by the matrix multiplication exponent , is a
central problem in algebraic complexity theory. The best upper bounds on
, leading to the state-of-the-art , have been
obtained via the laser method of Strassen and its generalization by Coppersmith
and Winograd. Recent barrier results show limitations for these and related
approaches to improve the upper bound on .
We introduce a new and more general barrier, providing stronger limitations
than in previous work. Concretely, we introduce the notion of "irreversibility"
of a tensor and we prove (in some precise sense) that any approach that uses an
irreversible tensor in an intermediate step (e.g., as a starting tensor in the
laser method) cannot give . In quantitative terms, we prove that
the best upper bound achievable is lower bounded by two times the
irreversibility of the intermediate tensor. The quantum functionals and
Strassen support functionals give (so far, the best) lower bounds on
irreversibility. We provide lower bounds on the irreversibility of key
intermediate tensors, including the small and big Coppersmith--Winograd
tensors, that improve limitations shown in previous work. Finally, we discuss
barriers on the group-theoretic approach in terms of "monomial"
irreversibility
Towards a Geometric Approach to Strassen's Asymptotic Rank Conjecture
We make a first geometric study of three varieties in (for each ), including the Zariski
closure of the set of tight tensors, the tensors with continuous regular
symmetry. Our motivation is to develop a geometric framework for Strassen's
Asymptotic Rank Conjecture that the asymptotic rank of any tight tensor is
minimal. In particular, we determine the dimension of the set of tight tensors.
We prove that this dimension equals the dimension of the set of oblique
tensors, a less restrictive class introduced by Strassen.Comment: Final version. Revisions in Section 1 and Section
Tensor rank is not multiplicative under the tensor product
The tensor rank of a tensor t is the smallest number r such that t can be
decomposed as a sum of r simple tensors. Let s be a k-tensor and let t be an
l-tensor. The tensor product of s and t is a (k + l)-tensor. Tensor rank is
sub-multiplicative under the tensor product. We revisit the connection between
restrictions and degenerations. A result of our study is that tensor rank is
not in general multiplicative under the tensor product. This answers a question
of Draisma and Saptharishi. Specifically, if a tensor t has border rank
strictly smaller than its rank, then the tensor rank of t is not multiplicative
under taking a sufficiently hight tensor product power. The "tensor Kronecker
product" from algebraic complexity theory is related to our tensor product but
different, namely it multiplies two k-tensors to get a k-tensor.
Nonmultiplicativity of the tensor Kronecker product has been known since the
work of Strassen.
It remains an open question whether border rank and asymptotic rank are
multiplicative under the tensor product. Interestingly, lower bounds on border
rank obtained from generalised flattenings (including Young flattenings)
multiply under the tensor product
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