@misc{indiciaeca5032b5c81f,
  title = {General Self-Adjoint Boundary Conditions for Certain Partial Differential Operators.},
  author = {J W Calkin},
  year = {1939},
  doi = {10.1073/pnas.25.4.201},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588287/},
  note = {Source identifier: 16588287}
}