The lattice (a 3-cube in this case) of all subfields of the finite field of order 230
In June 2003 we visited the Czech and Slovak republics with, what a pleasure to read, the rough guide at hand. While walking on the main street of the small town of Kežmarok, from the beautiful wooden Articular Church [with a capacity of nearly 1500 attendants!] to the former Thököly castle, my eye fell on a plaquette commemorating Hronec's 16 years as head of the renowned local grammar-school. The other mathematician, on the stamp, is Štefan Schwarz. He wrote a thesis on reducibility of polynomials over finite fields, which should explain the lattice in the middle [his supervisor Karel Petr appears on another stamp, at the top of this page]. Schwarz most important work was in semigroup theory. Some 35 years ago I enjoyed reading the classical book of A.H.Clifford & G.B.Preston on this subject, prompted by my first room-mate in Twente, Anton Jetten.