Singular eigenstates in the even(odd) length Heisenberg spin chain
  We study the implications of the regularization for the singular solutions on the even(odd) length spin-1/2 XXX chains in some specific down-spin sectors. In particular, the analytic expressions of the Bethe eigenstates for three down-spin sector have been obtained along with their numerical forms in some fixed length chains. For an even-length chain if the singular solutions \{\lambda_\alpha\} are invariant under the sign changes of their rapidities {\lambda_\alpha\}=\{-\lambda_\alpha\} , then the Bethe ansatz equations are reduced to a system of (M-2)/2 ((M-3)/2) equations in an even (odd) down-spin sector. For an odd N length chain in the three down-spin sector, it has been analytically shown that there exist singular solutions in any finite length of the spin chain of the form N= 3\left(2k+1\right) with k=1, 2, 3, \cdots. It is also shown that there exist no singular solutions in the four down-spin sector for some odd-length spin-1/2 XXX chains. 
Pulak Ranjan Giri, Tetsuo Deguchi
hep-th cond-mat.str-el math-ph math.MP nlin.SI