Let f ∈ C1[- π, π ], Define for \(n \in \mathbb{N}\), \(b_n = \int_{-\pi}^{\pi} f(t) sin(nt) dt\), which of the following is correct?
1
bn → 0 as n → ∞
2
nbn → 0 as n → ∞
3
The series \(\sum _{n=1}^{\infty} n^3 b_n^3\) is absolutely convergent.
4
All the above