Complex sentence: In 1955, Alfred Tarski showed that RRA is itself a variety, which however, as shown by Donald Monk in 1964, has no finite axiomatization, unlike RA which is finitely axiomatized by definition.
Split into simpler sentences: In 1955, Alfred Tarski showed that RRA is itself a variety. In 1964, Donald Monk showed that RRA has no finite axiomatization, unlike RA which is finitely axiomatized by definition.