Marco Casazza Alberto Ceselli Marc Nunkesser Efficient Algorithms for the Double Traveling Salesman Problem with Multiple Stacks. 7-10 2009 CTW http://www.lix.polytechnique.fr/ctw09/ctw09-proceedings.pdf#page=19 conf/colognetwente/2009 db/conf/colognetwente/colognetwente2009.html#CasazzaCN09