@misc{indiciaef98200cfbfa5,
  title = {Abstract Self-Adjoint Boundary Conditions.},
  author = {J W Calkin},
  year = {1938},
  doi = {10.1073/pnas.24.1.38},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588184/},
  note = {Source identifier: 16588184}
}