@article{article082a70de,
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 = {1993},
journal   = {Annales de l´Institut Henri Poincaré. Physique Theorique}
}