Computability and Logic

330 MODAL LOGIC AND PROVABILITY
27.2 Lemma. For any extension S of K,if⊢ S A → B, then ⊢ S □A → □B.
Proof: Suppose we have a proof of A → B. Then we can then extend it as follows:
(1) A → B G
(2) □(A → B) N(1)
(3) □(A → B) → (□A → □B) A
(4) □A → □B T(2), (3)
The annotations mean: G[iven], [by] N[ecessitation from step] (1), A[xiom], and
T[autological consequence of steps] (2), (3).
27.3 Lemma. ⊢ K (□A & □B) ↔ □(A & B), and similarly for more conjuncts.
Proof:
(1) (A & B) → A T
(2) □(A & B) → □A 25.2(1)
(3) □(A & B) → □B S(2)
(4) A → (B → (A & B)) T
(5) □A → □(B → (A & B)) 25.2(4)
(6) □(B → (A & B)) → (□B → □(A & B)) A
(7) (□A & □B) ↔ □(A & B) T(2), (3), (5), (6)
The first three annotations mean: T[autology], [by Lemma] 25.2 [from] (1), and
S[imilar to] (2).
Proof of Theorem 27.1: There are four assertions to be proved.
K is sound for the class of all models. Let W be any model, and write w |= A
for W, w |= A. It will be enough to show that if A is an axiom, then for all w we
have w |= A, and that if A follows by a rule from B 1 ,...,B n , and for all w we have
w |= B i for each i, then for all w we have w |= A.
Axioms. IfA is tautologous, the clauses of the definition of |= for ⊥ and →
guarantee that w |= A. As for axioms of the other kind, if w |= □(A → B) and w |=
□A, then for any v