4Krugman P. First nature, second nature and metropolitan location. Journal of Regional Science, 1993, 33: 129-144.
5O'Kelly M E, Bryan D, Shorin-kapov D et al. Hub network de- sign with single and multiple allocation: a computational study Location Science, 1996, 4( 3 ): 125-138.
6O'Kelly M E, Bryan D. Hub location with flow economies of scale. Transpn Res. B, 32(8): 605-616.