Georg Pick was born on August 10, 1859, and died on July 26, 1942, having spent much of his career as a professor at the German University of Prague.
He is most commonly remembered for a theorem concerning the area of lattice polygons, polygons in the plane whose vertices occur at points with integer coordinates: the area of such a polygon is numerically equal to one less than the number of lattice points in its interior plus half the number of lattice points on its boundary. Pick published his theorem in 1899, but it received scant attention until it was commented upon by Steinhaus in his 1969 text Mathematical Snapshots.
Part of the popularity of Pick’s Theorem is its elegance: it is simple to state, it is simple to discover (once told to expect a relationship involving area, lattice points in the interior, and lattice points on the boundary, initial explorations with rectangles suffice to generate linear relationships that suggest Pick’s result); and a formal proof [e.g., this one at Cut the Knot] is relatively straightforward.
Mathematicians with significant anniversaries during the week of July 20 – July 26:
July 20: Death anniversaries of John Playfair  (Playfair’s Axiom), Bernhard Riemann  (geometric foundations), and Andrei Andreyevich Markov  (probability, stochastic processes)
July 21: Birthday of John Leech  (Leech Lattice); death of Giovanni Frattini  (group theory)
July 22: Birthday of Wilhelm Bessel  (analysis), Gabriel Lamé  (differential geometry, proof of FLT for exponent 7), and Konrad Knopp  (analysis)
July 23: Death of Florence Nightingale David  (statistics) [her parents were friends of "the" Florence Nightingale]
July 24: Birthday of Errett Bishop  (Foundations of Constructive Analysis); death of Hans Hahn  (Hahn-Banach Theorem)
July 25: Birthday of Johann Listing  (topology)
July 26: Birthday of Kurt Mahler  (p-adic numbers, geometry of numbers); deaths of Gottlob Frege  (mathematical logic), Henri Lebesgue  (measure and integral), Georg Pick  (Pick’s Theorem), Raymond C. Archibald  (history of mathematics), and John Tukey  (mathematical statistics)