@article{UZERU_2013_1_a0,
     author = {T. A. Grigoryan and E. V. Lipacheva and V. H. Tepoyan},
     title = {On $\pi$-extensions of the semigroup $\mathbb{Z}_+$},
     journal = {Proceedings of the Yerevan State University. Physical and mathematical sciences},
     pages = {3--5},
     publisher = {mathdoc},
     number = {1},
     year = {2013},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/UZERU_2013_1_a0/}
}