Sparse random tensors : Concentration, regularization and applications

We prove a non-asymptotic concentration inequality for the spectral norm of sparse inhomogeneous random tensors with Bernoulli entries. For an order-k inhomogeneous random tensor T with sparsity p_max ≥ c log n/n, we show that parallel to ǁT - ETǁ = O(√np_max log^k-2(n)) with high probability. The optimality of this bound up to polylog factors is provided by an information theoretic lower bound. By tensor unfolding, we extend the range of sparsity to p_max ≥ c log n/n^m with 1 ≤ m ≤ k - 1 and obtain concentration inequalities for different sparsity regimes. We also provide a simple way to regularize T such that O(√np_max) concentration still holds down to sparsity p(max) ≥ c/n^m with k/2 ≤ m ≤ k - 1. We present our concentration and regularization results with two applications: (i) a randomized construction of hypergraphs of bounded degrees with good expander mixing properties, (ii) concentration of sparsified tensors under uniform sampling.