@article {Greif1999,
    AUTHOR = {Greif, Chen},
     TITLE = {Reduced systems for three-dimensional elliptic equations with
              variable coefficients},
   JOURNAL = {SIAM J. Matrix Anal. Appl.},
    VOLUME = {21},
      YEAR = {1999},
    NUMBER = {1},
     PAGES = {29--44}
}