•Proof Suppose
CONN is expressed by f, i.e. G ² f iff G is connected
•
•Let s’=s [ {s,t}
yk = : 9 x1, …, xk R(s,x1) Æ … Æ R(xk,t)
•The set G = {f} [ {y1, y2, …} is satisfiable (by compactness)
•Let G be a model: G ² f but there is no path from s to t, contradiction
•THIS PROOF IS INSSUFFICIENT OF US.
WHY ?