@article{SF_1979__17_134316,
     author = {W. Tonev},
     title = {Positive definite functions on discrete commutative  semigroups.},
     journal = {Semigroup forum},
     pages = {175--184},
     publisher = {mathdoc},
     volume = {17},
     year = {1979},
     zbl = {0398.20074},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__17_134316/}
}