The stoichiometric subspace of a reaction network is the real vector space spanned by its reaction
vectors (Boros and Hofbauer 2020). Thus
A mass-action system has its vector field in ,
so a trajectory starting at
remains in
. Its positive stoichiometric
compatibility class is
For the two-species network ,
is spanned by
, and every positive compatibility
class is an open line segment
with
. The total concentration is conserved.