1. Alexeev, R.B., Peller, V.V.: Badly approximable matrix functions and canonical factorizations. Indiana Univ. Math. J. 49, 1247–1285 (2000)
2. Conway, J.B.: A Course in Functional Analysis. Graduate Texts in Mathematics, 2nd edn. Springer, New York (1990)
3. Helson, H.: Lectures on Invariant Subspaces. Academic Press, New York (1964)
4. Peller, V.V.: Hankel Operators and Their Applications. Springer Monographs in Mathematics. Springer, New York (2003)
5. Peller, V.V.: Hankel operators and continuity properties of best approximation operators. Algebra i Analiz 2:1 (1990), 163–189 (English transl.: Leningrad Math. J. 2:1 (1991), 139–160)