studied the works of Wallis and Barrow; he and Leibniz became friends and tutored each other. Jacob developed important methods for integral and differential equations, coining the word integral. He and his brother were the key pioneers in mathematics during the generations between the era of Newton-Leibniz and the rise of Leonhard Euler.
Jacob liked to pose and solve physical optimization problems. His "catenary" problem (what shape does a clothesline take?) became more famous than the "tautochrone" solved by Huygens. Perhaps the most famous of such problems was the brachistochrone, wherein Jacob recognized Newton's "lion's paw", and about which Johann Bernoulli wrote: "You will be petrified with astonishment [that] this same cycloid, the tautochrone of Huygens, is the brachistochrone we are seeking." Jacob did significant work outside calculus; in fact his most famous work was the Art of Conjecture, a textbook on probability and combinatorics which proves the Law of Large Numbers, the Power Series Equation, and introduces the Bernoulli numbers. He is credited with the invention of polar coordinates (though Newton and Alberuni had also discovered them). Jacob also did outstanding work in geometry, for example constructing perpendicular lines which quadrisect a triangle.