 typically used when restarting Krylov solvers. Truncating a
tensor <A HREF="../BV/BV.html#BV">BV</A> V = (I otimes U) S to its leading columns amounts to keeping the
leading columns of S. However, to effectively reduce the size of the
decomposition, it is necessary to compress it in a way that fewer columns of
U are employed. This can be achieved by means of an update that involves the
<A HREF="../SVD/SVD.html#SVD">SVD</A> of the low-rank matrix [S_0 S_1 ... S_{d-1}], where S_i are the pieces of S.
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If 