Convergence of regular approximations to the spectra of singular fourth   order Sturm-Liouville problems
  We prove some new results which justify the use of interval truncation as a means of regularising a singular fourth order Sturm-Liouville problem near a singular endpoint. Of particular interest are the results in the so called lim-3 case, which has no analogue in second order singular problems. 
Malcolm Brown and Leon Greenberg and Marco Marletta