*Article* **A Plea for the Integration of Fractional Differential Systems: The Initial Value Problem**

**Nezha Maamri 1,\* and Jean-Claude Trigeassou <sup>2</sup>**


**\*** Correspondence: nezha.maamri@univ-poitiers.fr

**Abstract:** The usual approach to the integration of fractional order initial value problems is based on the Caputo derivative, whose initial conditions are used to formulate the classical integral equation. Thanks to an elementary counter example, we demonstrate that this technique leads to wrong freeresponse transients. The solution of this fundamental problem is to use the frequency-distributed model of the fractional integrator and its distributed initial conditions. Using this model, we solve the previous counter example and propose a methodology which is the generalization of the integer order approach. Finally, this technique is applied to the modeling of Fractional Differential Systems (FDS) and the formulation of their transients in the linear case. Two expressions are derived, one using the Mittag–Leffler function and a new one based on the definition of a distributed exponential function.

**Keywords:** initial value problem; fractional differential systems; fractional integral equation; infinitestate approach; Riemann–Liouville integral; frequency distributed exponential function
