%0 Journal Article %T Existence of Periodic Solution for a Nonlinear Fractional Differential Equation %A Belmekki Mohammed %A Nieto JuanJ %A Rodr¨ªguez-L¨®pez Rosana %J Boundary Value Problems %D 2009 %I Springer %X We study the existence of solutions for a class of fractional differential equations. Due to the singularity of the possible solutions, we introduce a new and proper concept of periodic boundary value conditions. We present Green's function and give some existence results for the linear case and then we study the nonlinear problem. %U http://www.boundaryvalueproblems.com/content/2009/324561