Hackbusch Conjecture on tensor formats
Resolves a foundational open problem in tensor network theory for extremal tree structures, providing theoretical bounds for practitioners choosing between tensor formats.
Proved Hackbusch's conjecture comparing the complexity of hierarchical (HF) and tensor train (TT) formats for representing tensors, showing that TT formats can be exponentially more efficient than HF formats for certain tensors.
We prove a conjecture of W. Hackbusch about tensor network states related to a perfect binary tree and train track tree. Tensor network states are used to present seemingly complicated tensors in a relatively simple and efficient manner. Each such presentation is described by a binary tree and a collection of vector spaces, one for each vertex of the tree. A problem suggested by Wolfgang Hackbusch and Joseph Landsberg is to compare the complexities of encodings, if one presents the same tensor with respect to two different trees. We answer this question when the two trees are extremal cases: the most "spread" tree (perfect binary tree), and the "deepest" binary tree (train track tree). The corresponding tensor formats are called hierarchical formats (HF) and tensor train (TT) formats, respectively.