@misc{indiciaefb71a02e0ca1,
  title = {Abstract Definite Boundary Value Problems.},
  author = {J W Calkin},
  year = {1940},
  doi = {10.1073/pnas.26.12.708},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588415/},
  note = {Source identifier: 16588415}
}