Lie was particularly pleased with this achievement: see Helgason 1994, p. 7. Helgason, Sigurdur (1994), "Sophus Lie, the Mathematician"(PDF), Proceedings of the Sophus Lie Memorial Conference, Oslo, August, 1992, Oslo: Scandinavian University Press, pp. 3–21.
The following discussion is based on Helgason 1994, pp. 4–5. Helgason, Sigurdur (1994), "Sophus Lie, the Mathematician"(PDF), Proceedings of the Sophus Lie Memorial Conference, Oslo, August, 1992, Oslo: Scandinavian University Press, pp. 3–21.