Define \( f(u) = u - \frac{u^5}{5} \) for every real number \( u \). If \( n \) is a positive integer, define \( b_n \) by \( b_1 = 1 \) and \( b_{n+1} = f(b_n) \). Find all values of \( n \) for which \( b_n < 0.1 \).