Unique positive solution for a fractional boundary value problem

In this work we consider the unique positive solution for the following fractional boundary value problem {D(0+)(alpha)u(t) = -f(t,u(t)), t is an element of [0, 1], u(0) = u'(0) =u'(1) = 0. Here alpha is an element of (2, 3] is a real number, D (0+) (alpha) is the standard Riemann-Liouville fractional derivative of order alpha. By using the method of upper and lower solutions and monotone iterative technique, we also obtain that there exists a sequence of iterations uniformly converges to the unique solution.