The Schr\"odinger equation with piercings
  We show that the spectrum of the Schr\"odinger equation in two or higher dimensions does not change when Dirichlet boundary conditions are enforced on a number of isolated points inside the original domain (piercings). We have obtained the analytical solution for spherically symmetric state of the $d$-dimensional simple harmonic oscillator pierced at the origin. Results for the case with multiple piercings are obtained numerically and agree with the theoretical prediction. In the case of a two dimensional parabolic quantum dot with two electrons and a single piercing in the origin we show that the energy of the ground state calculated to first order in perturbation theory goes over to the equivalent result in absence of piercing as the radius of the piercing becomes infinitesimal. Interestingly, we find that the leading finite size correction to the interaction energy is negative while the corresponding correction to the single particle energies is (as expected) positive. For a finite radius of the piercing and above a critical coupling, the first term dominates the second and the total energy of the dot with piercing is lower. Unfortunately the critical coupling found for this example is nonperturbative. Finally we study configurations allowing bound states in the continuum, showing that bound states survive to the insertion of piercings and that their energy is unchanged when the radius of the piercing vanishes. 
Paolo Amore