function mu =  shift2(D);
%% D is a 2x2 matrix and shift2 computes the smallest ev. of D
%%
   del = D(1,1)*D(2,2) - D(1,2)*D(2,1);
%% 
   s = 0.5*(D(1,1) + D(2,2));
if (s*s >= del)  
   if (s>0) 
       mu = s - sqrt(s*s - del) ;
   else 
       mu = s + sqrt(s*s - del) ;
   end
 else 
    mu = s + i*sqrt(del-s*s); 
 end
