Definition:Summation

Let $$\left({S, +}\right)$$ be an algebraic structure where the operation $$+$$ is an operation derived from, or arising from, the addition operation on the natural numbers.

Let $$\left({a_1, a_2, \ldots, a_n}\right) \in S^n$$ be an ordered $n$-tuple in $$S$$.

Then the composite is called the sum of $$\left({a_1, a_2, \ldots, a_n}\right)$$, and is written:


 * $$\sum \limits_{j=1}^n a_j = \left({a_1 + a_2 + \cdots + a_n}\right)$$

The sign $$\sum$$ is called the summation sign and sometimes referred to as sigma (as that's its name in Greek).

Alternatively:


 * $$\sum \limits_{1 \le j \le n} a_j = \left({a_1 + a_2 + \cdots + a_n}\right)$$

If $$\Phi \left({j}\right)$$ is a propositional function of $$j$$, then we can write:


 * $$\sum \limits_{\Phi \left({j}\right)} a_j = \text{ The sum of all } a_j \text{ such that } \Phi \left({j}\right) \text{ holds}$$.

Note that $$1 \le j \le n$$ is in fact a special case of such a propositional function, and that $$\sum \limits_{j=1}^n$$ is merely another way of writing $$\sum \limits_{1 \le j \le n}$$.

Thus, when it comes down to it, all instances of a summation can be expressed in terms of a propositional function.

If an infinite number of values of $$j$$ satisfy the propositional function $$\Phi \left({j}\right)$$, then the precise meaning of $$\sum \limits_{\Phi \left({j}\right)} a_j$$ is:


 * $$\sum \limits_{\Phi \left({j}\right)} a_j = \left({\lim_{n \to \infty} \sum_{\stackrel{R \left({j}\right)}{-n \le j < 0}} a_j}\right) + \left({\lim_{n \to \infty} \sum_{\stackrel{R \left({j}\right)}{0 \le j \le n}} a_j}\right)$$

provided that both limits exist. If either limit does fails to exist, then the infinite sum does not exist.

Note also that if more than one propositional function is written under the summation sign, they must all hold.

Historical Note
The notation $$\sum \limits_{j=1}^n a_j$$ was introduced by Joseph Fourier in 1820.