1. R. P. Boas, Entire functions, Academic Press Inc., New York, 1954.
2. J. D. Buckholtz, The Whittaker constant and successive derivatives of entire functions, J. Approximation Theory, 3 (1970), 194–212.
3. I. J. Schoenberg, On the zeros of successive derivatives of integral functions, Trans. Amer. Math. Soc, 40 (1936), 12–23.
4. J. M. Whittaker, Interpolatory function theory, Cambridge University Press, 1935.