J.J. Sakurai Modern Quantum Mechanics p. 74 It says, [A,H] = 0; H|a'> = Ea' |a'> where H is the Hamiltonian A is any observable |a'> is eigenket of A then, exp ( -iHt/h)|a'> = exp (-iEa't/h)|a'> where h is the reduced planck's constant. I want to know WHY ? and besides, I would like to ask what is generating function means? I have encountered xnew= x + dx pnew = p then generating function for infinitesimal translation is F(x,P) = x · P + p · dx How can I understand it ? Thank You ! I wish this time is readily to read !