© 1996 by London Mathematical Society
© The London Mathematical Society
Self-Adjoint Operators and Cones
School of Mathematical Sciences, University of Bath Bath BA2 7AY
Received 8 June 1994.
Suppose that K is a cone in a real Hilbert space
with K
= {0}, and that A:
is a self-adjoint operator which maps K into itself. If ||A|| is an eigenvalue of A, it is shown that it has an eigenvector in the cone. As a corollary, it follows that if ||A||n is an eigenvalue of An, then ||A|| is an eigenvalue of A which has an eigenvector in K. The role of the support-boundary of K in the simplicity of the principal eigenvalue ||A|| is investigated. If H is a separable Hilbert space, it is shown that ||A|| 
(A); that is, the spectral radius of A lies in the spectrum of A. When A is compact, we obtain a very elementary proof of the Krein-Rutman Theorem in the self-adjoint case without assuming that K
= {0}.