Frobenius Method for Computing Power Series Solutions of Linear Higher-Order Differential Systems

We consider the problem of computing regular formal solutions of systems of linear differential equations with analytic coefficients. The classical approach consists in reducing the system to an equivalent scalar linear differential equation and to apply the well-known Frobenius method. This transformation to a scalar equation is not necessarily relevant so we propose a generalization of the Frobenius method to handle directly square linear differential systems. Finally, we investigate the case of rectangular systems and show how their regular formal solutions can be obtained by computing those of an auxiliary square system.