In the physics literature, the spectral theorem is often stated by saying that a self-adjoint operator has an orthonormal basis of eigenvectors. Physicists are well aware, however, of the phenomenon of "continuous spectrum"; thus, when they speak of an "orthonormal basis" they mean either an orthonormal basis in the classic sense "or" some continuous analog thereof. In the case of the momentum operator formula_104, for example, physicists would say that the eigenvectors are the functions formula_105, which are clearly not in the Hilbert space formula_96. (Physicists would say that the eigenvectors are "non-normalizable.") Physicists would then go on to say that these "eigenvectors" are orthonormal in a continuous sense, where the usual Kronecker delta formula_107 is replaced by a Dirac delta function formula_108.