His most important discoveries in physics are his theory of light, colors and refraction, his theory of force, momentum and motion and his gravitational theory, all of which are still taught at highschools and universities today for undergraduates in physics. In mathematics his inventions are too many to report shortly, but apart from his invention of calculus, he contributed greatly to the theory of series and partial sums, Diophantine equations and numerical analysis, all of which today are basic material which any undergraduate mathematician has to learn. Isaac Newton was on occasions challenged by mathematicians on the continent on difficult mathematical problems, correspondence by letter