Notation for sum of products.

Question

I have a set of sucessive elements $\{a_1, a_2, a_3, a_4\}$ from an arithmetic progression and I need to sum all the possible product-combinations of two elements, i.e. $$ a_1\cdot a_2+a_1\cdot a_3+a_1\cdot a_4 +a_2\cdot a_3+a_2\cdot a_4+a_3\cdot a_4 $$ Is there a standard notation for this?

Thanks.

Answer 1

$$\sum_{1\le i\lt j\le4} a_ia_j$$

Answer 2

Yes, the sigma notation for this series is:

$$\begin{align} \sum_{j=1}^3\;\sum_{k=j+1}^4 a_j a_k \;&=\; a_1 a_2 + a_1 a_3 + a_1 a_4 + a_2 a_3 +a_2 a_4 + a_3 a_4 \\ & = \sum_{1\leq j < k \leq 4} a_j a_k \end{align}$$

Answer 3

I don't know of a specific notation, but your sum is

$$\frac12\left(\left(\sum a_i\right)^2-\sum a_i^2\right)$$