[Math] 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

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