13.4: 2, 7, 9, 12, 18, 28 (Optional: 5, 10, 13, 17)
13.5: 2, 7, 14, 15, 20, 21, 22 (Optional: 4, 12, 16) *Hint: for #20 calculate div(curl G).
13.6: 5, 11, 23, 27 (Optional: 7, 14, 19, 24, 31, 38)
13.7: 2, 3, 5, 9, 10 (Optional: 6, 7)
13.8: 5, 8, 10, 11 (Optional: 7, 9, 12)