constant radius of the Hubble sphere

can you set up a FLRW-model in which the Hubble sphere has a constant comoving radius? what would be needed for such a construction? bonus question: for which \(n\) is the equality \(\mathrm{i}^{n^2}=\pm\mathrm{i}^n\) with \(\mathrm{i^2}=-1\) fulfilled?