1. Introduction
Fractional calculus[6] [14] [15] is a natural generalization of calculus. Although its mathematical history is very long, it starts to play an important role in many branches of science during the last three decades. Itis well known that many of engineering physical phenomenon can be modelled more accurately and authentically using the fractional order differential equations, so there has been a growing interest in investigating * the solutions and properties of these equations[3] [4], [5] [9] [4] [5] [9] [12].