text: An astrophysical disk can be modeled as a fluid with negligible mass compared to the central object (e.g. a star) and with negligible pressure. We can suppose an axial symmetry such that . 
  Starting from the equations of movement in cylindrical coordinates : 
  The second line implies that the specific angular momentum is conserved. We can then define an effective potential  and so :
  We can apply a small perturbation  to the circular orbit : 
  So, 
  And thus : 
  We then note 
  In a circular orbit . Thus : 
  The frequency of a circular orbit is  which finally yields :