8Augustus Demorgan. The Elements of Algebra [M]. Second Edition. Printed for Taylor and Walton, London, 1837: 168.
9Elias Loomis. Elements of Analytical Geometry and of the Differential and Integral Calculus [M]. Nineteen Edition. Harper & Brothers Publishers, New York. 1865: 113.
10Augustus Demorgan. Elements of Arithmetic [M]. Fourth Edition. Printed for Taylor and Walton, London, 1839: 99.
