The Danish Mathematical Society's logo is a drawing by Th. Clausen from 1840 of a Hippocratean moon. The moon above has the same area as the concave polygon below.
Thomas Clausen (1801-85) was born in Sottrup. He studied astronomy at the observatory in Altona and in 1843 became the head of the observatory in Dorpat, now known as Tartu in Estonia. Clausen is known for a theorem with von Staudt on Bernoulli numbers, for the following formula for hypergeometric functions:
![\begin{displaymath}_3F_2\left[\matrix{2a,2b,a+b;x\cr 2a+2b,a+b+\frac12} ight]= \left({}_2F_1\left[\matrix{a,b;x\cr a+b+\frac12} ight] ight)^2 \end{displaymath}](hypergeometrisk.gif)
The original drawings of Clausen:
See also The MacTutor History of Mathematics archive.