Published online by Cambridge University Press: 17 September 2020
We consider the problem of finding two free export/import sets  $E^+$ and
$E^+$ and  $E^-$ that minimize the total cost of some export/import transportation problem (with export/import taxes
$E^-$ that minimize the total cost of some export/import transportation problem (with export/import taxes  $g^\pm $), between two densities
$g^\pm $), between two densities  $f^+$ and
$f^+$ and  $f^-$, plus penalization terms on
$f^-$, plus penalization terms on  $E^+$ and
$E^+$ and  $E^-$. First, we prove the existence of such optimal sets under some assumptions on
$E^-$. First, we prove the existence of such optimal sets under some assumptions on  $f^\pm $ and
$f^\pm $ and  $g^\pm $. Then we study some properties of these sets such as convexity and regularity. In particular, we show that the optimal free export (resp. import) region
$g^\pm $. Then we study some properties of these sets such as convexity and regularity. In particular, we show that the optimal free export (resp. import) region  $E^+$ (resp.
$E^+$ (resp.  $E^-$) has a boundary of class
$E^-$) has a boundary of class  $C^2$ as soon as
$C^2$ as soon as  $f^+$ (resp.
$f^+$ (resp.  $f^-$) is continuous and
$f^-$) is continuous and  $\partial E^+$ (resp.
$\partial E^+$ (resp.  $\partial E^-$) is
$\partial E^-$) is  $C^{2,1}$ provided that
$C^{2,1}$ provided that  $f^+$ (resp.
$f^+$ (resp.  $f^-$) is Lipschitz.
$f^-$) is Lipschitz.