Nikolai Lobachevsky

Lobachevsky's Geometrische Untersuchungen zur Theorie der Parallellinien (Berlin, 1840), the short book in which he set out non-Euclidean geometry: thirty-seven numbered theorems, from the definition of parallels and the angle of parallelism Π(p) to the trigonometry of the imaginary geometry. George Bruce Halsted's English translation, in the Open Court edition of 1914, with his preface, introduction, appendix on elliptic geometry and bibliography.