Fu et al. noted that the fundamental solution in
(3.45) is expressed as

where and are given by

To interpret their expression for the double layer kernel

we see that the following alternative to (3.87) is available:

From (3.86) and (3.88) we obtain

which can further be reduced to

where and are certain operators defined in Fu et al.[18]. In this way they could maintain the term in their expression for

(3.91) | |||

(3.92) |

are now written as

Substituting (3.17) into (3.93) and (3.94), one obtains

where

The multipole moments has three components and has one. Also, the multipole moments has nine components and has three. Thus, one can see that Fu et al.[18] use 4 and 12 multipole moments for the single- and double-layer potentials, respectively.

It is seen that the expression used by Fu et al.[18] is not
the only possibility in elastostatics which allows the use of
the FMM for Laplace's equation. For example one uses (3.89)
instead of (3.90) to have

Substituting (3.17) into (3.95) one obtains

where and are given by

The moments have 6 components (since ) and have 3. Hence in this case the number of multipole moments for the double-layer potential is 9 for a given pair of

On the other hand, in order to obtain our formulations for the single-
and double-layer potentials we first substitute (3.17)
into (3.86):

Noting the following identities

one sees that (3.96) is identical with (3.49).

In order to obtain our expression for the double-layer kernel we use
(3.87), instead of (3.88), with (3.49) to
have

Substituting (3.96) and (3.97) into (3.93) and (3.94) we obtain

where

The multipole moments has three components and has one. Also, the multipole moments has three components and has one. In our formulations for both the single- and double-layer potentials the number of the multipole moments is 4 and, moreover, and are the same as the multipole moments (3.53) and (3.54) for crack problems.

Notice that in this case the 4 moment formulation may not necessarily be three times faster than the 12 moment formulation. However, it is seen that our formulation is probably more efficient than Fu et al.'s one because the reduction of the number of the multipole moments leads to the reduction of M2L translations which dominate the performance of FMM.