example of a projective module which is not free
Let and be two nontrivial, unital rings and let . Furthermore let be a projection for . Note that in this case both and are (left) modules over via
where on the right side we have the multiplication in a ring .
Proposition. Both and are projective -modules, but neither nor is free.
Proof. Obviously is isomorphic (as a -modules) with thus both and are projective as a direct summands of a free module.
Assume now that is free, i.e. there exists which is a basis. Take any . Both and are nontrivial and thus in both and . Therefore in , but
This situation is impossible in free modules (linear combination is uniquely determined by scalars). Contradiction
. Analogously we prove that is not free.