Let W={w,u,v} and let the relation R on W be euclidean and symmetric;
Suppose R(w,u) and R(u,v).
-by symmetry we get R(u,v) -by euclidean we get R(v,w) and R(w,v).
Similarly, by euclidean we get R(u,u) & R(v,v) & R(w,w). In other words, every world has access to itself.
This looks identical to S5 which has a universal frame.
My three questions:
1: Does every world access itself like i think it does given the Set W and relation R on W?
2: if it does, are the two systems: KB5 and S5 identical or is S5 a sub logic of KB5?
3: or, is this just a coincidence that in this case the frame <W, R, v> is identical to a fully universal frame.
This is what I think:
This apparent reflexivity is just a coincidence given W and R; the two systems KB5 and S5 are not identical neither is S5 a sublogic of KB5.
S5 is definitely not a sublogic of KB5 (i.e. there are formulas provable in S5 that are not provable in KB5). Naturally, the two are not equivalent either.
Am i correct?
The apparent universality of this frame is throwing me off. Any ideas...