Arne Jensen, Gheorghe Nenciu
On the Fermi Golden Rule: Degenerate Eigenvalues
(205K, pdf)

ABSTRACT.  We review and further develop the framework in [10] of the 
stationary theory of resonances, arising by perturbation of either 
threshold, or embedded in the continuum, eigenvalues. While in [10] 
only non-degenerate eigenvalues were considered, here we add some results 
for the degenerate case. 
[10] A. Jensen and G. Nenciu, The Fermi Golden Rule and its form 
at thresholds in odd dimensions, 
Comm. Math. Phys. 261 (2006), 693--727.