1. W. K. Hayman, A generalisation of Stirling's Formula,Journal für reine und angewandte Mathematik, Bd. 169 (1956), 67–95.
2. E. C. Titchmarsh, The Theory of Functions (Second Edition), Oxford 1939.
3. G. Valiron, Lectures on the General Theory of Integral Functions, Chelsea 1949.