“…Kellerer [52], Rachev and Rüschendorf [67], or Villani [80], while unique characterizations of optimal maps for other costs have been investigated by Ahmad [3], Ambrosio and Rigot [6], Caffarelli [18], Gangbo and McCann [41], [42], [43], Gangbo andŚwiȩch [44], McCann [60], [61], Plakhov [66], Rüschendorf and Uckelmann [10], Uckelmann [78] in various geometries. Monge's cost c.x; y/ D jx yj in particular has attracted recent attention from Ambrosio [7] and Pratelli [9], Ambrosio, Kircheim and Pratelli [5], Caffarelli,Feldman and McCann [20], DePascale, Evans and Pratelli [30], Feldman and McCann [37], [36], and Trudinger and Wang [76] following the work of Evans and Gangbo [34] and Sudakov [75]. Although the free boundary problem we pose has not been much studied, other transportation problems in which one measure is fixed and the second is selected by a variational principle have been examined extensively in the context of dynamical problems since the work of Otto [65] and Jordan, Kinderlehrer and Otto [50], see also e.g.…”