@article{kellendonk_richard_2006, 
  title={Levinson's theorem for Schrödinger operators with point interaction: a topological approach}, 
  volume={39}, 
  DOI={10.1088/0305-4470/39/46/011}, 
  number={46}, 
  publisher={IOP Publishing}, 
  author={Kellendonk and Richard}, 
  year={2006}, 
  month={Nov}
  }