Norming Algebras and Automatic Complete Boundedness of Isomorphisms of Operator Algebras

We combine the notion of norming algebra introduced by Pop, Sinclair and Smith with a result of Pisier to show that if A_1 and A_2 are operator algebras, then any bounded epimorphism of A_1 onto A_2 is completely bounded provided that A_2 contains a norming C*-subalgebra. We use this result to give some insights into Kadison's Similarity Problem: we show that every faithful bounded homomorphism of a C*-algebra on a Hilbert space has completely bounded inverse, and show that a bounded representation of a C*-algebra is similar to a *-representation precisely when the image operator algebra \lambda-norms itself. We give two applications to isometric isomorphisms of certain operator algebras. The first is an extension of a result of Davidson and Power on isometric isomorphisms of CSL algebras. Secondly, we show that an isometric isomorphism between subalgebras A_i of C*-diagonals (C_i,D_i) (i=1,2) satisfying D_i \subseteq A_i \subseteq C_i extends uniquely to a *-isomorphism of the C*-algebras generated by A_1 and A_2; this generalizes results of Muhly-Qiu-Solel and Donsig-Pitts.