@article{701062,
     title = {Positive definite functions on Abelian semigroups},
     booktitle = {Abstracta. 4th Winter School on Abstract Analysis},
     series = {GDML\_Books},
     publisher = {Czechoslovak Academy of Sciences},
     address = {Praha},
     year = {1976},
     pages = {135-139},
     url = {http://dml.mathdoc.fr/item/701062}
}

Ressel, Paul. Positive definite functions on Abelian semigroups, dans Abstracta. 4th Winter School on Abstract Analysis, GDML_Books,  (1976), pp. 135-139. http://gdmltest.u-ga.fr/item/701062/