
Johann Carl Friedrich Gauss - "Princeps mathematicorum", died in Göttingen on February 23, 1855
- Carl Friedrich Gauss on MacTutor History of Mathematics archive
- Disquisitiones arithmeticae
- Quadratic reciprocity
- Gauss sums
The problem of distinguishing prime numbers from composite numbers and of resolving the latter into their prime factors is known to be one of the most important and useful in arithmetic. It has engaged the industry and wisdom of ancient and modern geometers to such an extent that it would be superfluous to discuss the problem at length... Further, the dignity of the science itself seems to require that every possible means be explored for the solution of a problem so elegant and so celebrated. [Disquisitiones Arithmeticae (1801) Article 329].
Finally, two days ago, I succeeded— not on account of my hard efforts, but by the grace of the Lord. Like a sudden flash of lightning, the riddle was solved. I am unable to say what was the conducting thread that connected what I previously knew with what made my success possible. [Mathematical Circles Squared: A Third Collection of Mathematical Stories and Anecdotes (1972) by Howard W. Eves]

Image: Grave of Gauss at Göttingen
In 1826, on the same day, February 23 (that's February 11 in old style) - Nikolai Ivanovich Lobachevsky reported to the session of the department of physics and mathematics at the University of Kazan on a geometry in which the fifth postulate was not true. The general consensus is that both Nikolai Lobachevsky and János Bolyai, working independently of each other around 1830, are to be credited with the discovery of non-Euclidean geometry. However, Lobachevsky's 1826 report appears to be the first time that non-Euclidean geometry results were publicly announced to the mathematical community.