An Extended Pruess Method for Sturm-Liouville Problems
  A new version of the piecewise approximation (Pruess) method is developed for calculating eigenvalues of Sturm-Liouville problems. The usual piecewise constant or piecewise linear potential approximations are replaced by translates of $2/cos^2(x)$, whose corresponding eigenvalue equation has elementary solutions. 
Robert Carlson
math.NA