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"It is very agreeable getting the angle-sum of a triangle = a straight angle"
ALS, one page, 5 x 8, The Manor House letterhead, March 12, 1918. Handwritten letter to Dr. Ludwik Silberstein, in full: "First & foremost let me thank you most warmly for 'solutio problematis marmalator'—I was in town for the week-end & thoroughly enjoyed the 'solution'—I do hope it does not mean that you have used up your month's jam ration in obtaining it? I think you should certainly add the matter of angle-measurement that you send me. It is very agreeable getting the angle-sum of a triangle = a straight angle. It seems to me to complete your work in a most interesting way." In fine condition.
Though perhaps best known for his work in philosophy, much of Russell's early life was dedicated to mathematics—An Essay on the Foundations of Geometry (1897), The Principles of Mathematics (1903), and Principia Mathematica (1910-13) were among his early, important works on the subject.