In 1871 Lie was awarded a scholarship to Christiania University. He also taught at Nissen’s Private Latin School. In July 1872 he received his Ph.D. During this period he developed the integration theory of partial differential equations now found in many textbooks, although rarely under his name. Lie’s results were found at the same time by Adolph Mayer, with whom he conducted a lively correspondence. Lie’s letters are a valuable source of knowledge about his development.