@article{1077477036,
author = {Levinson, Norman},
title = {The $\mathcal{L}$ -closure of eigenfunctions associated with self-adjoint boundary value problems},
journal = {Duke Math. J.},
volume = {19},
number = {1},
year = {1952},
pages = { 23-26},
language = {en},
url = {http://dml.mathdoc.fr/item/1077477036}
}
Levinson, Norman. The $\mathcal{L}$ -closure of eigenfunctions associated with self-adjoint boundary value problems. Duke Math. J., Tome 19 (1952) no. 1, pp. 23-26. http://gdmltest.u-ga.fr/item/1077477036/