@article{JIS_2009__12_8_a2,
     author = {Witu{\l}a,  Roman},
     title = {Ramanujan type trigonometric formulas: the general form for the argument \(\frac{2\pi}{7}\)},
     journal = {Journal of integer sequences},
     year = {2009},
     volume = {12},
     number = {8},
     zbl = {1201.11022},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JIS_2009__12_8_a2/}
}