Changes
* If <math>D</math> is not [[linear equivalence of graph divisors|linearly equivalent]] to an effective divisor, then <math> \rm{rank}(D) = -1</math>.
* Determining the rank of a graph divisor is '''NP'''-hard, even on simple undirected graphs <ref name="KT15"/>.
* The most well-known theorem about this rank function is the [[Riemann--Roch-theorem of Baker and Norine]] <ref name="BN07"/>.
=== References ===
