ABSTRACT. We study the central connection.problem for linear systems of equations with two singularities: one at the origin which is assumed to be regular-singular, and another one at infinity having a formal fundamental solution of only one level (in the Newton polygon).
INTRODUCTIONThe so-called centra/connection problem (CCP for short) has been investigated, in various degrees of generality, by many authors. Without claim of completeness, we have included the relevant papers in the list of references (for articles older than 1989 we used a literature search which A. Mehl [21], then a student in Ulm, made for his graduation paper).In particular, M. Kohno [14]-[18] studied the CCP for certain equations and obtained explicit formulas for the central connection coemcients. Here, we shall not only treat a case which is (slightly) more general than that considered by Kohno, but also follow a more direct approach to the problem (although the basic idea is the same): we consider an n-dimensional system of linear ordinary differential equations having a regular singularity at the origin and, as the only other singular point, an irregular singularity at infinity. For the time being, we restrict ourselves to the cases where the Newton polygon infinity only has one slope, or in other words, the formal fundamental solution is ~-summable for a single level ~ (see, e.g., [4] for the definition of ~-summability, resp., multisummability). (This assumption is, however, not the same as saying that we have all distinct eingenvalues at infinity, so the formal monodromy matrix is allowed to be nondiagonalizable.1991 Mathematics Subject Classifica~or~ 34A25, 44A10.