Numerical requirements for simulations of self gravitating and non-self gravitating disks
(highly abridged) We define three requirements for accurate simulations that attempt to model circumstellar disks and the formation of collapsed objects (e.g. planets) within them. First, we define a resolution requirement based on the wavelength for neutral stability of self gravitating waves in the disk. For particle based or grid based simulations, this criterion takes the form, respectively, of a minimum number of particles per critical (`Toomre') mass or maximum value of a `Toomre number', $T= \delta x/\lambda_T$, where the wavelength, $\lambda_T$, is the wavelength for neutral stability for waves in disks. We apply our criterion to particle simulations and find that in order to prevent numerically induced fragmentation of the disk, the Toomre mass must be resolved by a minimum of six times the average number of neighbor particles used. Second, we require that particle based simulations with self gravity use a variable gravitational softening. We show that using a fixed gravitational softening length can lead either to artificial suppression or enhancement of structure (including fragmentation) in a given disk, or both in different locations of the same disk. Third, we require that 3D SPH simulations resolve the disk's vertical structure with at least $\sim4$ particle smoothing lengths per scale height at the disk midplane and suggest that a similar criterion applies to grid based simulations. Failure to meet this criterion leads to underestimates in the midplane density of up to 30--50% at resolutions common in the literature.