@article{GDZPPN00126107X,
title = {Semigroups in the universal covering of SL(2).},
journal = {Semigroup forum},
volume = {43},
year = {1991},
pages = {33-44},
zbl = {0736.22005},
url = {http://dml.mathdoc.fr/item/GDZPPN00126107X}
}
Neeb, K.H. Semigroups in the universal covering of SL(2).. Semigroup forum, Tome 43 (1991) pp. 33-44. http://gdmltest.u-ga.fr/item/GDZPPN00126107X/