Kronecker powers of tensors and Strassen's laser method
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We answer a question, posed implicitly by Coppersmith-Winogrand and Buergisser et. al. and explicitly by Blaeser, showing the border rank of the Kronecker square of the little Coppersmith-Winograd tensor is the square of the border rank of the tensor for all q>2, a negative result for complexity theory. We further show that when q>4, the analogous result holds for the Kronecker cube. In the positive direction, we enlarge the list of explicit tensors potentially useful for the laser method. We observe that a well-known tensor, the 3x3 determinant polynomial regarded as a tensor, could potentially be used in the laser method to prove the exponent of matrix multiplication is two. Because of this, we prove new upper bounds on its Waring rank and rank (both 18), border rank and Waring border rank (both 17), which, in addition to being promising for the laser method, are of interest in their own right. We discuss "skew" cousins of the little Coppersmith-Winograd tensor and indicate whey they may be useful for the laser method. We establish general results regarding border ranks of Kronecker powers of tensors, and make a detailed study of Kronecker squares of tensors of dimensions (3,3,3). In particular we show numerically that for generic tensors in this space, the rank and border rank are strictly sub-multiplicative.
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