Gesztesy F., Simon B., Teschl G.
Zeros of the Wronskian and Renormalized Oscillation Theory
(62K, AMSTeX)

ABSTRACT.  For general Sturm-Liouville operators with separated boundary
conditions, we prove the following: If $E_{1,2}\in\Bbb R$
and if $u_{1,2}$ solve the differential equation $Hu_j=E_j u_j$, $j=1,2$
and respectively satisfy the boundary condition on the left/right, then
the dimension of the spectral projection $P_{(E_1, E_2)}(H)$ of
$H$ equals the number of zeros of the Wronskian of $u_1$ and $u_2$.