@misc{indiciaebbb79cf2b9e5,
  title = {Functions Whose Even Derivatives Have a Prescribed Sign.},
  author = {D V Widder},
  year = {1940},
  doi = {10.1073/pnas.26.11.657},
  url = {https://pubmed.ncbi.nlm.nih.gov/16577991/},
  note = {Source identifier: 16577991}
}