${\mathcal{T}}_h$

CG-algorithm

PCG-algorithm

$e_r$

ndiv

iters.

$e_r$

ndiv

iters.

33 × 33

1.16E−3

1.24E−5

84

1.82E−3

6.19E−5

2

65 × 65

5.04E−4

3.29E−6

170

6.40E−4

1.09E−5

2

129 × 129

1.88E−4

9.04E−7

346

2.25E−4

1.93E−6

2

257 × 257

9.25E−5

2.23E−7

695

9.93E−5

3.40E−7

2