Carl Ludwig Siegel was a German mathematician specialising in analytic number theory. He is known for, amongst other things, his contributions to the Thue–Siegel–Roth theorem in Diophantine approximation and the Siegel mass formula for quadratic forms. He was named as one of the most important mathematicians of the 20th century.André Weil, without hesitation, named Siegel as the greatest mathematician of the first half of the 20th century. Atle Selberg said of Siegel and his work:
He was in some ways, perhaps, the most impressive mathematician I have met. I would say, in a way, devastatingly so. The things that Siegel tended to do were usually things that seemed impossible. Also after they were done, they still seemed almost impossible. Wikipedia
in a 1964 letter to L. J. Mordell as quoted by [C. S. Yogananda, The Life and Times of Bourbaki, June 2015, Resonance, 556–559, http://www.ias.ac.in/article/fulltext/reso/020/06/0556-0559] (quote from p. 558)
[Lectures on Celestial Mechanics, https://books.google.com/books?id=mZtrCQAAQBAJ&pg=PA1] (p. 1)
[Lectures on the Geometry of Numbers, https://books.google.com/books?id=dyH4CAAAQBAJ&pg=PA6] (p. 6)
