@article{PORMA_2000__57_3_48899,
     author = {Carvalho, L.A.V. and Ladeira, L.A.C. and Martelli, M.},
     title = {Forbidden periods in delay differential equations.},
     journal = {Portugaliae mathematica},
     pages = {259--271},
     publisher = {mathdoc},
     volume = {57},
     number = {3},
     year = {2000},
     zbl = {0969.34064},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_2000__57_3_48899/}
}