Exact Renormalization Group in Large $N$

We apply the exact renormalization group formalism to compute the effective action and potential of the four dimensional O$(N)$ linear sigma model in large $N$. With a finite momentum cutoff in place, the model is well defined. In the naive continuum limit where the cutoff is taken to infinity, the effective action suffers from a tachyon, and the effective potential is unbounded from below and develops a negative imaginary part. These problems disappear once a small enough cutoff is restored. The effective potential of the naive continuum limit, obtained by analytic continuation, is given explicitly in terms of the Lambert $W$ function.