1. The problem is to understand why when $u_n=1$, we assume $u_{n-1}$ and $u_{n-2}$ are also 1. 2. This situation often arises in sequences defined by recurrence relations, where each term depends on previous terms. 3. For example, if a sequence is defined by $u_n = f(u_{n-1}, u_{n-2})$ and we find $u_n=1$, to check consistency or solve the relation, we often assume $u_{n-1}=1$ and $u_{n-2}=1$. 4. This assumption is valid if the sequence is constant or if initial conditions are set to 1, making the sequence terms equal. 5. It helps simplify the recurrence and verify if the constant sequence $u_n=1$ satisfies the relation. 6. In summary, assuming $u_{n-1}=1$ and $u_{n-2}=1$ when $u_n=1$ is a method to test if the sequence can be constant and to solve or analyze the recurrence relation.