@article{10_4153_CJM_1966_089_4,
     author = {Nakai, Mitsuru and Sario, Leo},
     title = {Construction of {Principal} {Functions} by {Orthogonal} {Projection}},
     journal = {Canadian journal of mathematics},
     pages = {887--896},
     year = {1966},
     volume = {18},
     number = {1},
     doi = {10.4153/CJM-1966-089-4},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1966-089-4/}
}