  A growing number of largely uncoordinated initiatives focus on research
software sustainability. A comprehensive mapping of the research software
sustainability space can help identify gaps in their efforts, track results,
and avoid duplication of work. To this end, this paper suggests enhancing an
existing schematic of activities in research software sustainability, and
formalizing it in a directed graph model. Such a model can be further used to
define a classification schema which, applied to research results in the field,
can drive the identification of past activities and the planning of future
efforts.
