1. G. H. Hardy and E. M. Wright, An introduction to the theory of numbers, 4th ed., Oxford Univ. Press, London, 1960.
2. Dini derivatives of continuous functions;Morse, Anthony P.;Proc. Amer. Math. Soc.,1954
3. Interscience Tracts in Pure and Applied Mathematics, No. 14;Niven, Ivan,1963
4. The Carus Mathematical Monographs, No. 11;Niven, Ivan,1956