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19 ธันวาคม
1783 : Birthdate of
Charles-Julien Brianchon who, in 1820 published
the nine-point circle theorem. Although this theorem has been independently discovered many times he gave the first complete proof and coined the phrase "nine-point circle."
1894 : Karl Pearson introduces the Pearson family of densities.
1908 : Scientific American offers "A $500 prize for a simple explanation of the fourth dimension."
20 ธันวาคม
1623 Wilhelm Schickard, in a letter to
Kepler, describes his calculating machine.
1648 :
Thommaso Ceva born.
21 ธันวาคม
1946 : The
Detroit news reports the Purdue yell, "E to X, DY, DX-E to the X, DX-Cosine, Secant, Tangent, Sine--Three Point One Four One Nine--Square Root, Cube Root, BTU---Slipstick, Slide Rule, Yea Purdue."
22 ธันวาคม
1669 : "In a dark night, in bed, without pen, ink, or paper, or anything equivalent, I did by memory extract the
square root of 3000,00000,00000,00000,00000,00000,00000,00000, which I found to be
1,77205,08075,68077,29353,fere, and did the next day commit it to writing."
John Wallis 
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1819 : Birthdate of
Pierre-Ossian Bonnet whose favorite field was differential geometry, a field opened by Euler, Monge, and Gauss, but lacking systematic treatment when Bonnet rook it up in the 1840s.
1869 : Birthdate of
Dimitry Fedorovich Egoorov, who worked mainly in function theory.
23 ธันวาคม
1751 :
Jacobi called this
the birthday of elliptic functions because on this day a work by the Italian mathematician
Fagnano, which had been sent to the Berlin Academy, was handed to
Euler for review. The study of this work led Euler to his important investigations of elliptic integrals and to the discovery of the addition theorem.