Gravitational Instability of Magnetized Gaseous Disks by Takenori Nakano and Takashi Nakamura (1978, 10.1093/pasj/30.4.671)

Publications of the Astronomical Society of Japan 30(4), 671–679. The original abstract states the perpendicular frozen-field threshold $\Sigma /B<1/(2\pi \sqrt{G})$ in Gaussian units and the pressure-stabilized short-wave branch.