  The c_2 invariant of a Feynman graph is an arithmetic invariant which detects
many properties of the corresponding Feynman integral. In this paper, we define
the c_2 invariant in momentum space and prove that it equals the c_2 invariant
in parametric space for overall log-divergent graphs. Then we show that the c_2
invariant of a graph vanishes whenever it contains subdivergences. Finally, we
investigate how the c_2 invariant relates to identities such as the four-term
relation in knot theory.
