ction purposes. See Helicopteros Nacionales de Colombia, S.A. v. Hall, 466 U.S. 408, 411-12, 416-18, 104 S.Ct. 1868, 80 L.Ed.2d 404 (1984); Bersch v. Drexel Firestone, Inc., 519 F.2d 974, 998 (2d Cir.1975) (buying and selling American securities is insufficient to estab- lish that defendant was “doing business” in the Un