  The $c_2$ invariants in all 4 different representations of the Feynman period
(parametric and dual parametric representations, position and momentum spaces)
coincide for all log-divergent graphs that satisfy the combinatorial condition
called duality admissibility. We check this condition for a good subspace of
graphs, for instance for all planar graphs. After the result in \cite{D3}, the
coincidence holds for all physically relevant graphs.
