@misc{indiciae55ed9fa8b65e,
  title = {On Self-Adjoint Differential Equations of Second Order.},
  author = {W Leighton},
  year = {1949},
  doi = {10.1073/pnas.35.11.656},
  url = {https://pubmed.ncbi.nlm.nih.gov/16578321/},
  note = {Source identifier: 16578321}
}