This equation computes the angular frequency term (`omega_0`) of the transfer function for a unity gain low pass filter implementation of the Sallen-Key topology. This electronic filter is a special case of a second-order unity gain filter version of the voltage-controlled voltage-source (VCVS) topology.
`omega_0` is derived from the resistances and capacitances (`R_1, R_2, C_1, C_21`) of the filter (see picture).