@article{JRAM_1976__288_151805,
     author = {D.D. Bonar and K. Barth},
     title = {An annular function whose derivative is not annular.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {146--151},
     publisher = {mathdoc},
     volume = {288},
     year = {1976},
     zbl = {0333.30019},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1976__288_151805/}
}