\[b_{n} = n^{2} - 4n + 9\]
\[b_{n} = 409:\]
\[n^{2} - 4n + 9 = 409\]
\[n^{2} - 4n - 400 = 0\]
\[D_{1} = 4 + 400 = 404\]
\[n_{1,2} = 2 \pm \sqrt{404}\text{.\ }\]
\[Ответ:не\ является.\]