Duality for frames in Krein spaces

A J†frame for a Krein space H is in particular a frame for H (in the Hilbert space sense). But it is also compatible with the indefinite inner†product of H, meaning that it determines a pair of maximal uniformly definite subspaces, an analogue to the maximal dual pair associated with an orthonormal basis in a Krein space. This work is devoted to study duality for J†frames in Krein spaces. Also, tight and Parseval J†frames are defined and characterized.