Nunes Rabello, Tania, Campos Vieira, Maria Cristina, y Lopes Frota, Cicero. 2003. Small vertical vibrations of strings with moving ends. Revista Matemática Complutense 16, 1, 179–206. DOI:https://doi.org/10.5209/rev_REMA.2003.v16.n1.16869