Accelerately Expanding Cosmologies in f(R,?,X) Theory

In this study, beginning and today expansion of universe are viewed in f(R,?,X) gravity. Field equations and their solutions of Friedmann-Lemaître-Robertson-Walker cosmologies with perfect fluid are obtained by considering f(R,?,X)=f_0 R+f_1 X^q-V(?) model. Validity of both f(R,?,X) gravity and f(R,?,X)=f_0 R+f_1 X^q-V(?) model for non-static space-time geometries is discussed by making use of the obtained matter dynamics results such as pressure and energy density. It is seen that in all obtained solutions by taking into account early and late period expansion, f function is a constant. This indicates that f(R,?,X) function is a first-order dependent function of Ricci scalar. When f(R,?,X)=f_0 R+f_1 X^q-V(?) model is considered together, it is understood that the obtained solutions could be reduced to ?-CDM model for f(R) gravity in limits of ??0 and X?0. The fact that the obtained results agree with expected situations supports. So, f(R,?,X) theory is a consistent theory of gravity.