Lumley 1994 observed that rho filters caused divergence during CG iteration. Although he did not use the actual half-derivative filter, his results indicated that the half-derivative filter should cause even more divergence than the rho filters he did use. While not yet confirmed, these results may have been due to the CG algorithm he used. Lumley used the CG method described by Hestenes and Steifel 1952 which can be unstable Fomel (1996). The results in this paper were obtained with the CG method described by Claerbout 1986.