Chris Kyle died on February 2, 2013.
In plane geometry , the einstein problem asks about the existence of a single prototile that by itself forms an aperiodic set of prototiles , that is , a shape that can tessellate space , but only in a nonperiodic way .. prototile. prototile. aperiodic set of prototiles. aperiodic set of prototiles. tessellate. tessellation. nonperiodic. aperiodic tiling. Such a shape is called an `` einstein '' -LRB- not to be confused with the physicist Albert Einstein -RRB- , a play on the German words ein Stein , meaning one tile .. Albert Einstein. Albert Einstein. Depending on the particular definitions of nonperiodicity and the specifications of what sets may qualify as tiles and what types of matching rules are permitted , the problem is either open or solved .. The einstein problem can be seen as a natural extension of the second part of Hilbert 's eighteenth problem , which asks for a single polyhedron that tiles Euclidean 3-space , but such that no tessellation by this polyhedron is isohedral .. isohedral. isohedral. Such anisohedral tiles were found by Karl Reinhardt in 1928 , but these anisohedral tiles all tile space periodically .. anisohedral tiles. anisohedral tiling. Karl Reinhardt. Karl Reinhardt ( mathematician )