“…The standard and typical approach to study the existence of solutions to the nonlinear fractional boundary value problems is the fixed point theory, see for example [1,4,16,35]. But, another well-developed, successful and recently more attracting method is the calculus of variation to investigate the existence of solutions to the differential equation with type of integer order and very recently fractional order, the readers are referred to [7,[11][12][13][17][18][19][20][21]32,33,36] and the references therein to attain more information about this approach.…”