A definition of frames for Krein spaces is proposed, which extends the notion of J-orthonormal basis of 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 [ , ], meaning that it determines a pair of maximal uniformly J-definite subspaces with different positivity, an analogue to the maximal dual pair associated to a J-orthonormal basis.Also, each J-frame induces an indefinite reconstruction formula for the vectors in H, which resembles the one given by a J-orthonormal basis.

We elaborate on the deviation of the Jordan structures of two linear relations that are finite-dimensional perturbations of each other. We compare their number of Jordan chains of length at least n. In the operator case, it was recently proved that the difference of these numbers is independent of n and is at most the defect between the operators. One of the main results of this paper shows that in the case of linear relations this number has to be multiplied by $$n+1$$ n + 1 and that this bound is sharp. The reason for this behavior is the existence of singular chains. We apply our results to one-dimensional perturbations of singular and regular matrix pencils. This is done by representing matrix pencils via linear relations. This technique allows for both proving known results for regular pencils as well as new results for singular ones.