Generalized Hanson-Wright Inequality for Random Tensors

The Hanson-Wright inequality is an upper bound for tails of real quadratic forms in independent random variables. In this work, we extend the Hanson-Wright inequality for the Ky Fan k-norm for the polynomial function of the quadratic sum of random tensors under Einstein product. We decompose the quadratic tensors sum into the diagonal part and the coupling part. For the diagonal part, we can apply the generalized tensor Chernoff bound directly. But, for the coupling part, we have to apply decoupling method first, i.e., decoupling inequality to bound expressions with dependent random tensors with independent random tensors, before applying generalized tensor Chernoff bound again to get the the tail probability of the Ky Fan $k$-norm of the coupling part sum of independent random tensors. At the end, the generalized Hanson-Wright inequality for the Ky Fan k-norm for the polynomial function of the quadratic sum of random tensors can be obtained by the combination of the bound from the diagonal sum part and the bound from the coupling sum part.