@inbook{Duff+1956+98+124,
url = {https://doi.org/10.3138/9781442653993-006},
title = {V. Self-Adjoint Elliptic Equations},
booktitle = {Partial Differential Equations},
author = {G.F.D. Duff},
publisher = {University of Toronto Press},
address = {Toronto},
pages = {98--124},
doi = {doi:10.3138/9781442653993-006},
isbn = {9781442653993},
year = {1956},
lastchecked = {2026-05-07}
}