The Compatibility of the Gluon Reggeization with the s-channel Unitarity
  Recently the non-forward BFKL kernel for interaction of two Reggeized gluons in the antisymmetric colour octet state in the $t$-channel was obtained in the next-to-leading order. It gives the possibility to check in this order the bootstrap condition for this kernel, appearing as the requirement of the compatibility of gluon Reggeization with the $s$-channel unitarity. 
V.S. Fadin, R. Fiore, M.I. Kotsky