1. R. Courant, Differential and Integral Calculus II, Wiley, New York, 1936, p. 317.
2. J. Edwards, Treatise on Integral Calculus II, Chelsea, New York, 1922, p. 325.
3. G. M. Fikhtengol'ts, Fundamentals of Mathematical Analysis II, Pergamon, London, 1965, p. 151.
4. P. Franklin, Methods of Advanced Calculus, McGraw-Hill, New York, 1944, p. 270.