On undecidable propositions of formal mathematical systems (1934)
Solomon Feferman, John W Dawson, Stephen Cole Kleene, Gregory Martin Moore, Robert M Solovay, Jean van HEIJENOORT · 2001
Abstract A formal mathematical system is a system of symbols together with rules for employing them. The individual symbols are called undefined terms. Formulas are finite sequences of the undefined terms. There shall be defined a class of formulas called meaningful formulas, and a class of meaningful formulas called axioms. There may be a finite or infinite number of axioms.