Gravitational Collapse of Circularly Symmetric Stiff Fluid with   Self-Similarity in 2+1 Gravity
  Linear perturbations of homothetic self-similar stiff fluid solutions, $S[n]$, with circular symmetry in 2+1 gravity are studied. It is found that, except for those with $n = 1$ and $n = 3$, none of them is stable and all have more than one unstable mode. Hence, {\em none of these solutions can be critical}. 
A.Y. Miguelote, N. A. Tomimura and Anzhong Wang