@article{10_4064_fm220_3_5,
     author = {Neil Hindman and Dona Strauss and Yevhen Zelenyuk},
     title = {Longer chains of idempotents in $\beta G$},
     journal = {Fundamenta Mathematicae},
     pages = {243--261},
     publisher = {mathdoc},
     volume = {220},
     number = {3},
     year = {2013},
     doi = {10.4064/fm220-3-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/fm220-3-5/}
}