  We propose a fast algorithm for mode rank truncation of the result of a
bilinear operation on 3-tensors given in the Tucker or canonical form. If the
arguments and the result have mode sizes n and mode ranks r, the computation
costs $O(nr^3 + r^4)$. The algorithm is based on the cross approximation of
Gram matrices, and the accuracy of the resulted Tucker approximation is limited
by square root of machine precision.
