A nowhere-zero 
-flow
on a graph 
consists of an orientation of its edges
and an integer value 
assigned to each graph edge such that 
and the sum of the values on edges
directed into each graph vertex equals the sum on
edges directed out of it (Tutte 1954). Existence of
such a flow is independent of the chosen orientation.
For positive integer 
,
the number of nowhere-zero 
-flows on a graph is given by its
flow polynomial.
