Reference on Pontryagin space

I’m looking for a reference telling me that in Pontryagin indefinite spaces, a symmetric and closed operator KKK on π1−\pi_1-space has a maximal invariant negative semi-definite subspace which is of dimensions 11, and hence it has at least 11 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.

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