Estimates in corona theorems for some subalgebras of H^infty

We obtain estimates in the corona theorem for the algebra of analytic functions in the unit disc whose nth derivative is bounded, and its subalgebras defined by the boundary continuity of the nth derivative. The corona theorem for such algebras is trivial and known, but corona theorem with estimates is new and much harder to prove. As an auxiliary result of independent interest we get the continuity of the best estimate in such algebras (including the classical case of the algebra $H^\infty$ of bounded analytic functions). We also show that for a fixed $n$ the best estimate is the same for all algebras we consider.