# Definition:Series/Number Field

## Definition

Let $S$ be one of the standard number fields $\R$, or $\C$.

Let $\sequence {a_n}$ be a sequence in $S$.

The series is what results when $\sequence {a_n}$ is summed to infinity:

$\ds \sum_{n \mathop = 1}^\infty a_n = a_1 + a_2 + a_3 + \cdots$

### Real Series

A real series $S_n$ is the limit to infinity of the sequence of partial sums of a real sequence $\sequence {a_n}$:

 $\ds S_n$ $=$ $\ds \lim_{N \mathop \to \infty} \sum_{n \mathop = 1}^N a_n$ $\ds$ $=$ $\ds \sum_{n \mathop = 1}^\infty a_n$ $\ds$ $=$ $\ds a_1 + a_2 + a_3 + \cdots$

### Complex Series

A complex series $S_n$ is the limit to infinity of the sequence of partial sums of a complex sequence $\sequence {a_n}$:

 $\ds S_n$ $=$ $\ds \lim_{N \mathop \to \infty} \sum_{n \mathop = 1}^N a_n$ $\ds$ $=$ $\ds \sum_{n \mathop = 1}^\infty a_n$ $\ds$ $=$ $\ds a_1 + a_2 + a_3 + \cdots$

## Historical Note

Much of the original work on series of real and complex numbers was done by Leonhard Paul Euler.

The main bulk of the work to placed the concept on a rigorous footing was done by Carl Friedrich Gauss, Niels Henrik Abel‎ and Augustin Louis Cauchy.