Minimal number of singular fibers in a Lefschetz fibration

There exists a (relatively minimal) genus g Lefschetz fibration with only o ne singular fiber over a closed (Riemann) surface of genus h iff g greater than or equal to 3 and h greater than or equal to 2. The singular fiber can be chosen to be reducible or irreducible. Other results are that every Deh n twist on a closed surface of genus at least three is a product of two com mutators and no Dehn twist on any closed surface is equal to a single commu tator.