@article{Griffith2013,
author = {Griffith, D.A. and Fischer, M.M.},
title= {Constrained variants of the gravity model and spatial dependence: Model specification and estimation issues},
year = {2013},
doi = {10.1007/s10109-013-0182-7},
URL = {https://doi.org/doi:10.1007/s10109-013-0182-7},
volume = {15},
number = {3},
pages = {291‒317},
}