  We study planar two-dimensional quantum systems on a lattice whose
Hamiltonian is a sum of local commuting projectors of bounded range. We
consider whether or not such a system has a zero energy ground state. To do
this, we consider the problem as a one-dimensional problem, grouping all sites
along a column into "supersites"; using $C^*$-algebraic methods (Bravyi and
Vyalyi), we can solve this problem if we can characterize the central elements
of the interaction algebra on these supersite. Unfortunately, these central
elements may be very complex, making brute force impractical. Instead, we show
a characterization of these elements in terms of matrix product operators with
bounded bond dimension. This bound can be interpreted as a bound on the number
of particle types in lattice theories with bounded Hilbert space dimension on
each site. Topological order in this approach is related to the existence of
certain central elements which cannot be "broken" into smaller pieces without
creating an end excitation. Using this bound on bond dimension, we prove that
several special cases of this problem are in NP, and we give part of a proof
that the general case is in NP. Further, we characterize central elements that
appear in certain specific models, including toric code and Levin-Wen models,
as either product operators in the Abelian case or matrix product operators
with low bond dimension in the non-Abelian case; this matrix product operator
representation may have practical application in engineering the complicated
multi-spin interactions in the Levin-Wen models.
