Bostock, Linda; Chandler, Suzanne; Chandler, F. S. (1979). Pure Mathematics 2 (in ஆங்கிலம்). Nelson Thornes. p. 462. ISBN978-0-85950-097-5. Thus a cubic equation has either three real roots... or one real root...
wolfram.com
mathworld.wolfram.com
Weisstein, Eric W. "Stationary Point". mathworld.wolfram.com (in ஆங்கிலம்). Retrieved 2020-07-27.