Is the $O(3)~\sigma$ Model with the Hopf Term Exactly Equivalent to a   Higher Spin Theory?
  We write down a local $CP_1$ model involving two gauge fields, which is exactly equivalent to the O(3) $\sigma$ model with the Hopf term. We impose the $CP_1$ constraint by using the gaussian representation of the delta function. For the coefficient of the Hopf term, $\theta = {\pi \over 2s}$, 2s being an integer, we show that the resulting model is exactly equivalent to an interacting theory of spin-$s$ fields. Thus we conjecture that there should be a fixed point in the spin-$s$ theory near which it is exactly equal to the $\sigma$ model. 
T.R.Govindarajan, R.Shankar, N.Shaji and M.Sivakumar