  The goal of this research is to devise guaranteed defense policies that allow
to protect a given region from the entrance of smart mobile invaders by
detecting them using a team of defending agents equipped with identical line
sensors. By designing cooperative defense strategies that ensure all invaders
are detected, conditions on the defenders' speed are derived. Successful
accomplishment of the defense task implies invaders with a known limit on their
speed cannot slip past the defenders and enter the guarded region undetected.
The desired outcome of the defense protocols is to defend the area and
additionally to expand it as much as possible. Expansion becomes possible if
the defenders' speed exceeds a critical speed that is necessary to only defend
the initial region. We present results on the total search time, critical
speeds and maximal expansion possible for two types of novel pincer-movement
defense processes, circular and spiral, for any even number of defenders. The
proposed spiral process allows to detect invaders at nearly the lowest
theoretically optimal speed, and if this speed is exceeded, it also allows to
expand the protected region almost to the maximal area.
