# Infinite series, definition

Tao breaks the definition into two pieces (reasons discussed on flip side). I present the 2^{nd} part first. See the flip side for the initial definition of a *formal series*.

### Convergence of series, definition

Let \( \sum_{n=m}^{\infty} a_n \) be a formal infinite series. For any integer \( N \ge m \), we define the N^{th} partial sum \( S_N \) of this series to be

\( S_N \) is thus a real number.

If the sequence [...] converges to some limit \( L \), then we say that the infinite series \( \sum_{n=m}^{\infty} a_n \) is *convergent*, and *converges *to \( L \). We then write

and say that \( L \) is the sum of the infinite series \( \sum_{n=m}^{\infty} a_n \). If the sequence of partial sums \( (S_N)_{N=m}^{\infty} \) diverge, then we say that the infinite series \( \sum_{n=m}^{\infty} a_n \) is *divergent*, and we [...].