Hi guys I found this on Google Books: http://books.google.dk/books?id=v5v...resnum=5&ved=0CCwQ6AEwBA#v=onepage&q=&f=false Here they say that whenever we write the Hamiltonian for a non-interacting system in its eigenbasis, then we have that [tex] A(\nu, \omega) = 2\pi \delta(\omega-\varepsilon_\nu). [/tex] How can this statement be proven? Do you have any hints for this? Any help will be greatly appreciated. Sincerely, Niles.