Guillaume de l'Hôpital

You might see the name "l'Hôpital" spelled "l'Hospital", which is how the Marquis himself spelled it. When the French decided to change their spelling a bit, his name got changed too.

L'Hôpital served in the military for a while before he turned to mathematics. He studied under Johann Bernoulli and became a very able mathematician. In 1696 he wrote the very first textbook on differential calculus. In this book, he included the now famous rule that he had learned from Bernoulli< about calculating limits of functions of the form f(x)/g(x) where the numerator and denominator both have limit 0. Although he acknowledged his indebtedness to Bernoulli for this technique, the rule nevertheless became known as l'Hôpital's rule.