S. Toppila, P. Lounesto: On a conjecture of Hellerstein, Shen and Williamson.
Ann. Univ. Mariae Curie-Sklodowska, Sect. A 15 (1986), 267-268.

J.M. Anderson, S. Toppila: The growth of the spherical derivative of a
meromorphic function of finite lower order, J. London Math. Soc. (2) 27
(1983), no. 2, 289--305.

James Milne Anderson, Paul Erdos, Allan Pinkus, Oved Shisha:
The closed linear span of $`\x`sp k$--$c`sb k`\`sp `infty`sb 1$
J. Approx. Theory 43 (1985), no. 1, 75--80.