On non-definite Sturm-Liouville problems with two turning points
  This is an inaugural study of the Dirichlet problem associated with a regular non-definite Sturm-Liouville equation in the case of two turning points. We give a priori lower bounds on the Richardson numbers associated with this problem thereby complementing pioneering results by Atkinson and Jabon (1984) in the one turning point case 
Mervis Kikonko and Angelo B. Mingarelli
math.CA