Gerald Teschl
Renormalized Oscillation Theory for Dirac Operators
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ABSTRACT.  Oscillation theory for one-dimensional Dirac operators with separated boundary
conditions is investigated. Our main theorem reads: If $\lambda_{0,1}\in
\mathbb R$
and if $u,v$ solve the Dirac equation $H u= \lambda_0 u$, $H v= \lambda_1
v$ (in
the weak sense) and respectively satisfy the boundary condition on the
left/right,
then the dimension of the spectral projection $P_{(\lambda_0, \lambda_1)}(H)$
equals the number of zeros of the Wronskian of $u$ and $v$. As an
application we
establish finiteness of the number of eigenvalues in essential spectral gaps of
perturbed periodic Dirac operators.