On a class of elliptic systems in ℝ n . Costa, David G. — 1994 Electronic Journal of Differential Equations (EJDE) [electronic only]
Existence of positive solutions for semi-positone systems. Dramé, Abdou K.; Costa, David G. — 2008 Electronic Journal of Differential Equations (EJDE) [electronic only]
On a variational approach to existence and multiplicity results for semipositone problems. Costa, David G.; Tehrani, Hossein; Yang, Jianfu — 2006 Electronic Journal of Differential Equations (EJDE) [electronic only]
Existence and multiplicity results for nonlinear elliptic problems in ℝ N with an indefinite functional. Costa, David G.; Guo, Yuxia; Ramos, Miguel — 2002 Electronic Journal of Differential Equations (EJDE) [electronic only]