 Łukasiewicz and his students made the study of propositional calculi very much their own: the results obtained between 1920 and 1930 were published in a 1930 joint paper of Łukasiewicz and Tarski, “Untersuchungen über den Aussagenkalkül”. Work proceeded on both classical (bivalent) and many-valued calculi. The clearest and most complete demonstration of how Łukasiewicz in his maturity treated classical propositional calculus comes in his 1929 student textbook, based on lecture notes, Elements of Mathematical Logic. The system, following Frege, is based on implication \((C)\) and negation \((N)\) alone, with the elegant axiom set