(p ^ (!p or q)) implies q but is not equivalent to q.
in VanDallen's "Logic and structure", he defines a "duality mapping" d from PROP to PROP as follows(note that by !phi i mean negation of phi):
d(phi)=phi for atomic phi.
d(phi ^ psi)=d(phi) or d(psi)
d(phi or psi)=d(phi) ^ d(psi)
then he proves the following theorem(duality theorem):[note by p == q, i mean p and q are equivalent]
phi == psi iff d(phi) == d(psi) [in fluent English, if two propositions are equivalence, then so are their duals.]
Now my question:
We know that (p ^ (!p or q)) == q. Then why their duals are not so? i.e., why the following does not hold? : (p or (!p ^ q)) == q.