Complex sentence: In mathematics, the tunnel number of a knot is a knot invariant, given by the minimal number of arcs (called tunnels) that must be added to the knot so that the complement becomes a handlebody.
Split into simpler sentences: In mathematics, the tunnel number of a knot is a knot invariant. It is given by the minimal number of arcs (called tunnels) that must be added to the knot so that the complement becomes a handlebody.