Consider an nth-order linear ordinary differential equation

Suppose the αj(x) are holomorphic for x ∈ S, 0 < x0 ≤ |x| < ∞, where S is an open sector of the complex plane with vertex at the origin and positive central angle not exceeding π. Suppose as x → ∞ in each closed subsector of S. The problem of finding a basis for the solutions of (1) can be reduced by a sequence of transformations (see, for example, Gilbert (1)) to the problem of finding a fundamental matrix for a system of the form

where q is a non-negative integer, A(x) is an m by m matrix which is holomorphic for x ∈ S, x0 < ≤ |x| < ∞, and as x → ∞ in each closed subsector of S. If m ≥ 2, A0 is an m by m matrix with elements all zero except for l's on the upper off-diagonal, and the elements of Ar for r ≥ 1 are all zero except possibly in the last row. (There is one such problem (2) for each root, μ of the equation with multiplicity m ≥ 2. Recall that the coefficient αn−r, 0 the first term in the asymptotic expansion of the coefficint αn−r(x) of equation (1).) We denote the elements of the last row of Ar by 1 ≤ k ≤ m. The system (2) has an irregular singular point at infinity.