@article{SF_1994__49_2_135336,
     author = {D.V. Dekov},
     title = {HNN extensions of semigroups.},
     journal = {Semigroup forum},
     pages = {83--88},
     publisher = {mathdoc},
     volume = {49},
     number = {2},
     year = {1994},
     zbl = {0803.20039},
     url = {http://geodesic.mathdoc.fr/item/SF_1994__49_2_135336/}
}