Isomorphisms, Moduli, and Cohomological Dimension for Twisted Triangular Banach Algebras

Abstract

We introduce and study twisted triangular Banach algebras T_sigma(A,B;X), built from Banach algebras A,B, a Banach A-B bimodule X, and a pair of automorphisms sigma=(sigma_A,sigma_B). This construction extends the classical triangular framework by incorporating twisted module actions on the off-diagonal block. We obtain a complete isomorphism classification: T_sigma is isomorphic to T_tau precisely when the diagonal twists are conjugate, the bimodule admits an (alpha,beta)-equivariant isomorphism, and a shear cocycle satisfies a natural identity. In the case of group algebras, the classification detects conjugacy classes inside Aut(G), yielding new dynamical invariants absent from the untwisted setting. On the homological side, we establish sharp bounds for the Hochschild cohomological dimension and deduce that T_sigma is amenable only when X=0 and both A,B are amenable. Thus twisting enriches the classical theory while preserving extension-theoretic control of cohomology.