The role of rudimentary relations in complexity theory Hugo Volger — 1984 Mémoires de la Société Mathématique de France
Preservation Theorems for Limits of Structures and Global Sections of Sheaves of Structures. Hugo Volger — 1979 Mathematische Zeitschrift
A unifying approach to theorems on preservation and interpolation for binary relations between structures. Hugo Volger — 1981 Archiv für mathematische Logik und Grundlagenforschung