$k=10$

$k=20$

Mesh

Error

Order

Mesh

Error

Order

40 × 40

0.0108

40 × 40

0.0029

80 × 80

8.1990e−04

3.9831

80 × 80

3.0197e−04

3.5145

160 × 160

6.3034e−05

3.8346

160 × 160

3.4628e−05

3.2369

$320\times 320$

3.9823e−06

4.0563

$320\times 320$

6.9882e−06

2.3506

$640\times 640$

2.5763e−07

3.9859

$640\times 640$

2.3137e−07

4.9610