In this paper we provide a way to handle some symmetric fractional boundary-value problems. Indeed, first, we consider some system of fractional equations. We introduce the existence and uniqueness of solutions of the systems of equations and we show that they are entire functions of the spectral parameter. In particular, we show that the solutions are at most of order 1/2. Moreover we share the integration by parts rule for vector-valued functions that enables us to obtain some symmetric equations. These symmetries allow us to handle \(2-\) sequential and \(4-\) sequential fractional boundary-value problems. We provide some expansion formulas for the bilinear forms of the solutions of \(2-\) sequential and \(4-\) sequential fractional equations which admit us to impose some unusual boundary conditions for the solutions of fractional differential equations. We show that the systems of eigenfunctions of \(2-\) sequential and \(4-\) sequential fractional boundary value problems are complete in both energy and mean. Furthermore, we study on the zeros of solutions of \(2-\) sequential fractional differential equations. At the end of the paper we show that \(6-\) sequential fractional differential equation can also be handled as a system of equations and hence almost all the results obtained in the paper can be carried for such boundary-value problems.
