@article{JRAM_1981__321_152316,
     author = {Robert Shelton},
     title = {Aperiodic words on three symbols.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {195--209},
     publisher = {mathdoc},
     volume = {321},
     year = {1981},
     zbl = {0441.05007},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1981__321_152316/}
}