1. [1] Balser (W.), Jurkat (W. B.), Lutz (D. A.).— A general theory of invariants for meromorphic differential equations ; Part I, formal invariants, Funkcialaj Ekvacioj, 22, p. 197-221, (1979).
2. [2] Balser (W.), Jurkat (W. B.), Lutz (D. A.).— On the reduction of connection problems for differential equations with an irregular singular point to ones with only regular singularities, I. SIAM J. Math. Anal., 12(5), p. 691-721, (1981).
3. [3] Deligne (P.).— Equations différentielles à points singuliers réguliers, Lecture Notes in Mathematics, 163, Springer-Verlag, (1970).
4. [4] Duval (A.), Loday-Richaud (M.).— Kovacic’s algorithm and its application to some families of special functions, AAECC, 3, p. 211-246, (1992)
5. [5] Écalle (J.).— Les fonctions résurgentes, tome III : l’équation du pont et la classification analytique des objets locaux, Publications Mathématiques d’Orsay, n ∘ 85-05, (1985).