a) Schematic representation of the node splitting mechanism. The cartoon is two-dimensional but it is meant to illustrate a three-dimensional arrangement of sintered grains and pores between them. The large blue dots indicate the center of the nodes and the blue arrows symbolize the (out of plane) channels emanating from them. The large red arrow indicates the evolution associated with sintering. b) Example of graph transformations that leave the graph topological invariant (genus) unchanged while reducing the mean coordination number. In this cartoon, extra node/branch combinations (represented in red) are inserted at different places in the graph without altering the genus. By definition, nodes must always be present at the extremities of a branch (the dotted segments are intended to symbolize the unspecified continuation of the graph).