@misc{miscf31e504a,
title   = {The four positive vortices problem: regions of chaotic behavior and the non-integrability},
author = {Castilla, Maria Stella Amorim Coutinho and Moauro, Vinicio and Negrini, Piero and Oliva, Waldyr Muniz},
year = {1992},
publisher = {IME-USP}
}