TY  - RPRT
TI  - General Self-Adjoint Boundary Conditions for Certain Partial Differential Operators.
AU  - J W Calkin
PY  - 1939
DO  - 10.1073/pnas.25.4.201
UR  - https://pubmed.ncbi.nlm.nih.gov/16588287/
ID  - 16588287
ER  -