@article{JIS_2010__13_6_a0,
     author = {Beck,  Matthias and Zaslavsky,  Thomas},
     title = {Six little squares and how their numbers grow},
     journal = {Journal of integer sequences},
     year = {2010},
     volume = {13},
     number = {6},
     zbl = {1230.05062},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/JIS_2010__13_6_a0/}
}