  Given a conjunctive Boolean network (CBN) with $n$ state-variables, we
consider the problem of finding a minimal set of state-variables to directly
affect with an input so that the resulting conjunctive Boolean control network
(CBCN) is controllable. We give a necessary and sufficient condition for
controllability of a CBCN; an $O(n^2)$-time algorithm for testing
controllability; and prove that nonetheless the minimal controllability problem
for CBNs is NP-hard.
