Graph-Grammars: An Algebraic Approach - Hartmut Ehrig; Michael Pfender; Hans Jürgen Schneider (SWAT, 1973)

Diagram characterization. - Hartmut Ehrig; Wolfgang Kühnel; Michael Pfender (Category Theory Applied to Computation and Control, 1974)

Primitive Recursion, Equality, and a Universal Set. - Michael Pfender; K. Kröplin; D. Pape (Math. Struct. Comput. Sci., 1994)

Arithmetical Foundations - Excerpt. - Michael Pfender (CoRR, 2013)