“…Results on the existence and multiplicity of solutions for nonlinear boundary value problems of fractional differential equations in a fractional derivative space can be found in [16,17,18,19,33,41]. Ledesma [20,21,22] and others discuss variational problems via fractional derivatives and integrals, with important techniques, such as the Mountain Pass Theorem, Nehari manifolds, critical point theory and fibering maps (see [17,23,24,36,37,40,42]).…”