A method for finding upper and lower bounds for the fundamental eigenvalue in special eigenvalue problems is presented. The method is systematic and is shown to provide convergence from above and below to the exact eigenvalue under certain conditions. The method is based on the relatively well-known enclosure or comparison theorem of Collatz, and makes use of a power series to approximate the eigenfunction. The method is applied to two examples concerning the critical-elastic buckling load of variable-section columns with pinned ends. Results for the first example compare well with the exact solution, which is known; the second example is presented as an addition to the literature.