- The sequence of positive numbers \(u _ { 1 } , u _ { 2 } , u _ { 3 } , \ldots\), is given by
$$u _ { n + 1 } = \left( u _ { n } - 3 \right) ^ { 2 } , \quad u _ { 1 } = 1 .$$
- Find \(u _ { 2 } , u _ { 3 }\) and \(u _ { 4 }\).
- Write down the value of \(u _ { 20 }\).