MR0577846
Balser, W. and Jurkat, W. B. and Lutz, D. A.
W. BALSER, W. B. JURKAT and D. A. LUTZ
A general theory of invariants for meromorphic differential equations. {II}. Proper invariants
Fako de l'Funkcialaj Ekvacioj Japana Matematika Societo. Funkcialaj Ekvaciog. Serio Internacia
22
1979
257--283
http://fe.math.kobe-u.ac.jp/FE/Free/vol22/fe22-3-3.pdf
http://fe.math.kobe-u.ac.jp/FE/FE_pdf_with_bookmark/FE21-30-en_KML/fe22-257-283/fe22-257-283.pdf
http://www.ams.org/mathscinet-getitem?mr=MR0577846
fe22-257-283
1979
A General Theory of Invariants for Meromorphic Differential Equations; Part II, Proper Invariants
BALSER, W.; JURKAT, W. B.; LUTZ, D. A.
BALSER, W.; JURKAT, W. B.; LUTZ, D. A.