Linear differential equations on P1 and root systems

In this paper, we study the Euler transform on linear ordinary differential operators on P1. The spectral type is the tuple of integers which count the multiplicities of local formal solutions with the same leading terms. We compute the changes of spectral types under the action of the Euler transform and show that the changes of spectral types generate a transformation group of a Z-lattice which is isomorphic to a quotient lattice of a Kac–Moody root lattice with the Weyl group as the transformation group.