@article{10_1017_S0017089500001841,
     author = {Kelman, Robert B. and Jr, Chester A. Koper},
     title = {Least squares approximations for dual trigonometric series},
     journal = {Glasgow mathematical journal},
     pages = {111--119},
     year = {1973},
     volume = {14},
     number = {2},
     doi = {10.1017/S0017089500001841},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500001841/}
}