Which of the following conditions is necessary for a series ∑n=1∞an\sum_{n=1}^{\infty} a_n∑n=1∞an to converge?
limn→∞an=0\lim_{n \to \infty} a_n = 0limn→∞an=0
∑n=1∞∣an∣\sum_{n=1}^{\infty} |a_n|∑n=1∞∣an∣ converges
The sequence ana_nan is monotonic
The partial sums SnS_nSn are unbounded