Beyond Lazy Training for Over-parameterized Tensor Decomposition

Over-parametrization is an important technique in training neural networks. In both theory and practice, training a larger network allows the optimization algorithm to avoid bad local optimal solutions. In this paper we study a closely related tensor decomposition problem: given an l-th order tensor in (Rd)otimes l of rank r (where rll d), can variants of gradient descent find a rank m decomposition where m > r? We show that in a lazy training regime (similar to the NTK regime for neural networks) one needs at least m = Omega(dl-1), while a variant of gradient descent can find an approximate tensor when m = O^*(r2.5llog d). Our results show that gradient descent on over-parametrized objective could go beyond the lazy training regime and utilize certain low-rank structure in the data.