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We can also consider block variants of preconditioners for accelerated
methods. Block methods are normally feasible if the problem domain is
a Cartesian product grid; in that case a natural division in lines (or
planes in the 3-dimensional case), can be used for blocking, though
incomplete factorizations are not as effective in the 3-dimensional
case; see for instance Kettler [134].
In such a blocking scheme for Cartesian product grids,
both the size and number of the
blocks increases with the overall problem size. Another type of block
method will be mentioned in §.