APA
Heerman K. E., National Bureau of Economic Research & Sheldon I. M. (2018). Gravity and comparative advantage. Cambridge: NBER.
Chicago
Heerman Kari E.R, National Bureau of Economic Research and Sheldon Ian M. 2018. Gravity and comparative advantage. Cambridge: NBER.
Harvard
Heerman K. E., National Bureau of Economic Research and Sheldon I. M. (2018). Gravity and comparative advantage. Cambridge: NBER.
MLA
Heerman Kari E.R, National Bureau of Economic Research and Sheldon Ian M. Gravity and comparative advantage. Cambridge: NBER. 2018.