${\mathcal{T}}_h$

CG-algorithm

PCG-algorithm

$e_r$

ndiv

iters.

$e_r$

ndiv

iters.

33 × 33

3.84E−2

1.65E−3

9

6.87E−2

4.55E−3

4

65 × 65

1.65E−2

3.25E−4

18

8.50E−2

3.03E−3

3

129 × 129

1.18E−2

1.22E−4

34

5.99E−2

1.08E−3

3

257 × 257

7.57E−3

3.38E−5

67

8.90E−2

6.89E−4

2