In the invex problems, it is required to consider a same function for the objective and constraint functions. Because of this restriction, various difficulties are encountered in applications related to optimality and duality properties. To improve this situation, in this work, the invexity with respect to different functions ( i) is proposed to study optimality conditions and duality for nonlinear and multiobjective programming problems with or without differentiability. Under this generalized invexity or its extensions assumptions, new Kuhn-Tucker (Fritz-John) type necessary and sufficient...