First move the order parameter term to the right-hand side:Here , , so the integrating factor obeysChoose the irrelevant positive constant . Multiplying the equation by givesThus a Sturm-Liouville theory eigenvalue formulation with the requested eigenvalue isThis is the Sturm-Liouville formulation with Bessel order as eigenvalue. The spectral parameter is the squared order of the Bessel function, with treated as fixed. This differs from the also familiar formulation in which is the eigenvalue.
The factor is singular at the origin. Consequently the integral defining cannot literally use lower limit zero; take a positive reference point , obtain , and absorb into the normalization. The differential identity holds for . Boundary conditions and admissibility at zero and at the outer endpoint are additional ingredients needed to make a particular self-adjoint spectral problem.