@article{MOMA_1970__74_177515,
     author = {Gert Sabidussi},
     title = {Idempotency of {Circuit-induced} {Exchange} {Operators.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {330--335},
     publisher = {mathdoc},
     volume = {74},
     year = {1970},
     zbl = {0239.05130},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1970__74_177515/}
}