@misc{indiciae9fcae94de240,
  title = {Principal Quadratic Functionals and Self-Adjoint Second-Order Differential Equations.},
  author = {W Leighton},
  year = {1949},
  doi = {10.1073/pnas.35.4.192},
  url = {https://pubmed.ncbi.nlm.nih.gov/16588881/},
  note = {Source identifier: 16588881}
}