We study a game on a graph G played by r {\it revolutionaries} and s
{\it spies}. Initially, revolutionaries and then spies occupy vertices. In each
subsequent round, each revolutionary may move to a neighboring vertex or not
move, and then each spy has the same option. The revolutionaries win if m of
them meet at some vertex having no spy (at the end of a round); the spies win
if they can avoid this forever.
Let σ(G,m,r) denote the minimum number of spies needed to win. To
avoid degenerate cases, assume |V(G)|\ge r-m+1\ge\floor{r/m}\ge 1. The easy
bounds are then \floor{r/m}\le \sigma(G,m,r)\le r-m+1. We prove that the
lower bound is sharp when G has a rooted spanning tree T such that every
edge of G not in T joins two vertices having the same parent in T. As a
consequence, \sigma(G,m,r)\le\gamma(G)\floor{r/m}, where γ(G) is the
domination number; this bound is nearly sharp when γ(G)≤m.
For the random graph with constant edge-probability p, we obtain constants
c and c′ (depending on m and p) such that σ(G,m,r) is near the
trivial upper bound when r<clnn and at most c′ times the trivial lower
bound when r>c′lnn. For the hypercube Qd with d≥r, we have
σ(G,m,r)=r−m+1 when m=2, and for m≥3 at least r−39m spies are
needed.
For complete k-partite graphs with partite sets of size at least 2r, the
leading term in σ(G,m,r) is approximately k−1kmr
when k≥m. For k=2, we have
\sigma(G,2,r)=\bigl\lceil{\frac{\floor{7r/2}-3}5}\bigr\rceil and
\sigma(G,3,r)=\floor{r/2}, and in general 2m3r−3≤σ(G,m,r)≤m(1+1/3)r.Comment: 34 pages, 2 figures. The most important changes in this revision are
improvements of the results on hypercubes and random graphs. The proof of the
previous hypercube result has been deleted, but the statement remains because
it is stronger for m<52. In the random graph section we added a spy-strategy
resul