1. Barnsley, M., Bessis, D., and Moussa, P., J. Math. Phys. 20, 535 (1979).
2. Szegö, G., Orthogonal Polynomials, American Mathematical Society Colloquium Publications, Vol. 23, 1939.
3. Julia, G., J. Math. Pur. Appl. 4, 47 (1918). Myrberg, P.J., J. Math. Pur. Appl. 41, 339 (1962).
4. Collet, P. and Eckmann, J.P., ?Iterated Maps on the Interval as Dynamical System?, Progess in Physics, Vol. 1, Birkhäuser, 1980.
5. Misiurewicz, M., to appear in Publ. Math. IHES 53, see also lecture note at 1980 Les Houches Summer School. After this work was finished, we received a preprint by M.F. Barnsley, J.S. Geronimo, and A.N. Harrington, Georgia Institute of Technology, Atlanta (1981), in which a direct proof is given.