Guillaume de L'Hôpital was a French mathematician who wrote the first textbook on differential calculus, Analyse des infiniment petits pour l'intelligence des lignes courbes (1696). This contains the rule, now known as L'Hôpital's Rule, for finding the limit of a rational function whose numerator and denominator tend to zero at a point. Along with Isaac Newton, Gottfried Leibniz, and Jacob Bernoulli, de L'Hôpital was among the first to solve the brachistochrone problem.