4 A sequence of complex numbers is defined by
$$u _ { 1 } = 1 + \mathrm { i } \quad \text { and } \quad u _ { n + 1 } = \mathrm { i } u _ { n } ( n = 1,2,3 , \ldots )$$
- Find \(u _ { 2 } , u _ { 3 } , u _ { 4 } , u _ { 5 }\) and \(u _ { 6 }\).
- Describe the behaviour of the sequence.
- Hence evaluate \(\sum _ { n = 1 } ^ { 73 } u _ { n }\).