Within a low-dimensional subspace \mathcal{L} of \Complex^n it can be feasible to locate the maximum x and minimum y of r. Repeating that for an increasing chain \mathcal{L}_1 \subset \mathcal{L}_2 \subset \cdots produces two sequences of vectors: x_1, x_2, \ldots and y_1, y_2, \dotsc such that x_j, y_j \in \mathcal{L}_j and
