@article{10_1007_s10492_015_0097_3,
     author = {Yao, Siqin and Sun, Jiong and Zettl, Anton},
     title = {The {Sturm-Liouville} {Friedrichs} extension},
     journal = {Applications of Mathematics},
     pages = {299--320},
     publisher = {mathdoc},
     volume = {60},
     number = {3},
     year = {2015},
     doi = {10.1007/s10492-015-0097-3},
     mrnumber = {3419964},
     zbl = {06486913},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.1007/s10492-015-0097-3/}
}