I'm looking for a reference telling me that in Pontryagin indefinite spaces, a symmetric and closed operator $K$ on $\pi_1-$space has a maximal invariant negative semi-definite subspace which is of dimensions $1$, and hence it has at least $1$ negative semi-definite eigenvalue.
The paper I'm reading quotes "Introduction to the Spectral Theory of Operators in Spaces with an Indefinite Metric" of I. S. Iohvidov, M. G. Krein, and H. Langer, but I don't have access to this book. Can someone give me another book with the same result?
Thank you.
