I saw this equation in a literature review recently talking about the Toomre criterion for gravitational instability:
Given here in section 2.1.1: https://arxiv.org/pdf/1801.06117.pdf, viz.
But I am not seeing how they got the period to be in the denominator from my own workings i get it in the numerator:
What am I misunderstanding here? How did they get the orbital period in the denominator ?