@article{MS2_1990__66_1_167105,
     author = {J\"orgen Backelin},
     title = {On the number of semigroups of natural numbers.},
     journal = {Mathematica Scandinavica},
     pages = {197--215},
     year = {1990},
     volume = {66},
     number = {1},
     zbl = {0741.11006},
     url = {http://geodesic.mathdoc.fr/item/MS2_1990__66_1_167105/}
}