environ vocabulary TARSKI; begin reserve x,y,z,u,N,M,X,Y,Z for set; :: theorem TARSKI:1 ---- everything is a set canceled; :: as obvious :: ---- extensionality theorem :: TARSKI:2 (for x holds x in X iff x in Y) implies X = Y; :: singletons & unordered pairs definition let y; func { y } means :: TARSKI:def 1 x in it iff x = y; let z; func { y, z } means :: TARSKI:def 2 x in it iff x = y or x = z; commutativity; end; canceled 2; definition let X,Y; pred X c= Y means :: TARSKI:def 3 x in X implies x in Y; reflexivity; end; definition let X; func union X means :: TARSKI:def 4 x in it iff ex Y st x in Y & Y in X; end; canceled 2; :: ---- regularity theorem :: TARSKI:7 x in X implies ex Y st Y in X & not ex x st x in X & x in Y; scheme Fraenkel { A()-> set, P[set, set] }: ex X st for x holds x in X iff ex y st y in A() & P[y,x] provided for x,y,z st P[x,y] & P[x,z] holds y = z; :: ordered pairs definition let x,y; func [x,y] equals :: TARSKI:def 5 { { x,y }, { x } }; end; canceled; definition let X,Y; pred X,Y are_equipotent means :: TARSKI:def 6 ex Z st (for x st x in X ex y st y in Y & [x,y] in Z) & (for y st y in Y ex x st x in X & [x,y] in Z) & for x,y,z,u st [x,y] in Z & [z,u] in Z holds x = z iff y = u; end; :: Alfred Tarski :: Ueber unerreichbare Kardinalzahlen, :: Fundamenta Mathematicae, vol.30 (1938), pp.68-69 :: Axiom A. (Axiom der unerreichbaren Mengen). Zu jeder Menge N gibt es :: eine Menge M mit folgenden Eigenschaften : :: A1. N in M; :: A2. ist X in M und Y c= X, so ist Y in M; :: A3. ist X in M und ist Z die Menge, die alle Mengen Y c= X und keine :: andere Dinge als Element enthaelt, so, ist z in M; :: A4. ist X c= M und sind dabei die Menge X und M nicht gleichmaechtig, :: so ist X in M. :: also :: Alfred Tarski :: On Well-ordered Subsets of any Set, :: Fundamenta Mathematicae, vol.32 (1939), pp.176-183 :: A. For every set N there exists a system M of sets which satisfies :: the following conditions : :: (i) N in M :: (ii) if X in M and Y c= X, then Y in M :: (iii) if X in M and Z is the system of all subsets of X, then Z in M :: (iv) if X c= M and X and M do not have the same potency, then X in M. theorem :: TARSKI:9 ex M st N in M & (for X,Y holds X in M & Y c= X implies Y in M) & (for X st X in M ex Z st Z in M & for Y st Y c= X holds Y in Z) & (for X holds X c= M implies X,M are_equipotent or X in M);