On the Treves Criterion for the Boussinesq and other GD Hierarchies
  This is an addition to our paper (Lett.Math.Phys 65,2003,187-197) where an analogue of the Treves criterion for the first integrals of the KdV hierarchy was suggested for the Boussinesq hierarchy and its necessity was proven. In the present paper, it is proven that there exists the second, ``conjugated'', group of tests. Besides, the relationship between our method and the B\"acklund transformation method recently suggested by Morosi and Pizzocchero is discussed. 
L. A. Dickey