An operator A defined by a matrix can be written as something like: A = Ʃi,jlei><ejl <eilAlej> How does this representation translate to a continous basis, e.g. position basis, where operators are not matrices but rather differential operators etc. Can we still write for e.g. the kinetic energy operator T: T = ∫∫dr dr' lr'><rl <r'lTlr> ? Or how would T be represented.