@article{SF_1977__15_134248,
     author = {M. Gould},
     title = {An easy proof of {Ponizovski's} theorem.},
     journal = {Semigroup forum},
     pages = {181--182},
     year = {1977},
     volume = {15},
     zbl = {0381.20050},
     url = {http://geodesic.mathdoc.fr/item/SF_1977__15_134248/}
}