Remarks on interior transmission eigenvalues, Weyl formula and branching   billiards
  The paper contains the Weyl formula for the counting function of the interior transmission problem when the latter is parameter-elliptic. Branching billiard trajectories are constructed, and the second term of the Weyl asymptotics is estimated from above under some conditions on the set of periodic billiard trajectories. 
Evgeny Lakshtanov, Boris Vainberg
math-ph math.DS math.MP