Grigoryan, T.; Lipacheva, E.; Tepoyan, V. ON $\pi$-EXTENSIONS OF THE SEMIGROUP $Z_+$. Proc. YSU A: Phys. Math. Sci. 2013, 47 (1 (230), 3-5. https://doi.org/10.46991/PYSUA.2013.47.1.003.