Generalized Hamilton's Principle with Fractional Derivatives
  We generalize Hamilton's principle with fractional derivatives in Lagrangian $L(t,y(t),{}_0D_t^\al y(t),\alpha)$ so that the function $y$ and the order of fractional derivative $\alpha$ are varied in the minimization procedure. We derive stationarity conditions and discuss them through several examples. 
Teodor M. Atanackovic, Sanja Konjik, Ljubica Oparnica, Stevan   Pilipovic
math.FA