M. Langer and A. Luger,
On norms in indefinite product spaces,
Oper. Theory Adv. Appl. 198 (2010), 259–264
Abstract:
In a Krein space various norms can be defined by choosing different underlying
fundamental decompositions. In this note we consider this dependence explicitly
and draw the conclusion that — even in a Pontryagin space — there
does not exist a natural choice motivated by minimizing properties.