If SN=∑n=1NanS_N = \sum_{n=1}^{N} a_nSN=∑n=1Nan, what condition is necessary for ∑n=1∞an\sum_{n=1}^{\infty} a_n∑n=1∞an to exist?
The sequence of partial sums SNS_NSN must be bounded
The sequence of partial sums SNS_NSN must have a finite limit
The sequence ana_nan must decrease monotonically
The sequence ana_nan must have a finite limit