[1] P. Binding and H. Volkmer, Oscillation theory for Sturm-Liouville
problems with indefinite coefficients, Proc. Royal Soc. Edinburgh A131
(2001), 989-1002.
[2] X. Cao, Q. Kong, H. Wu and A. Zettl, Sturm-Liouville problems
whose leading coefficient function changes sign, Canadian J. Math. 55
(2003), 724-749.
[3] X. Cao, Q. Kong, H. Wu and A. Zettl, Geometric aspects of
Sturm-Liouville problems, III, Level surfaces of the n-th eigenvalue,
J. Comp. Appl. Math. 28 (2007), 176-193.
[4] M. Eastham, Q. Kong, H. Wu and A. Zettl, Inequalities among
eigenvalues of Sturm-Liouville problems, J. Inequalities & Appl. 3
(1999), 25-43.
[5] W. Everitt, M.Möller and A. Zettl, Discontinuous Dependence of
the n-th Sturm-Liouville Eigenvalue, In General Inequalities edited by
C. Bandle, W. Everitt, L. Losonszi & W. Walter, Birkhauser, 1997.
[6] C. Fulton, On generating theorems and conjectures in spectral
theory with computer assistance, Spectral, Theory and Computational
Methods of Sturm-Liouville Problems (Knoxville, TN, 1996), 285-299,
Lecture Notes in Pure & Appl. Math. 191, Dekker, New York, 1997.
[7] Q. Kong, H. Wu and A. Zettl, Dependence of the n-th
Sturm-Liouville eigenvalue on the problem, J. Differential Equations
156 (1999), 328-354.
[8] Q. Kong, H. Wu and A. Zettl, Geometric aspects of Sturm-Liouville
problems, I, Structures on spaces of boundary conditions, Proc. Royal
Soc. Edinburgh 130A (2000), 561-589.
[9] G. Wang, Z. Wang and H. Wu, Computing the indices of
Sturm-Liouville eigenvalues for coupled boundary conditions (the
EIGENIND-SLP codes), J. Comp. Appl. Math. 220 (2008), 490-507.
[10] Z. Wang and H. Wu, The index problem for eigenvalues for coupled
boundary conditions and Fulton\'s conjecture, Monatsh. Math. 157
(2009), 177-191.