Is there a way to get expression for squared double sum?
$$\left(\sum\limits_{i=1}^n \sum\limits_{j=1}^n a_i a_j\right)^2 = \left(\sum\limits_{i=1}^n \sum\limits_{j=1}^n a_i a_j\right)\left(\sum\limits_{k=1}^n \sum\limits_{l=1}^n a_k a_l\right) = ?$$
Something like expression for single squared sum, given here What is the square of summation?.
Thanks in advance.
Edit:
I managed to get next formula by guessing (looking expression for $n=2$):
\begin{align} \left(\sum\limits_{i=1}^n \sum\limits_{j=1}^n a_i a_j\right)^2 &= \sum\limits_{i=1}^n \sum\limits_{j=1}^n a_i^2 a_j^2 + \sum\limits_{i=1}^n \sum\limits_{j \neq l}^n a_i^2 a_j a_l + \sum\limits_{i \neq k}^n \sum\limits_{j=1}^n a_i a_k a_j^2 + \sum\limits_{i \neq k}^n \sum\limits_{j \neq l}^n a_i a_k a_j a_l \\ &= \sum\limits_{i=1}^n \sum\limits_{j=1}^n a_i^2 a_j^2 + 2\sum\limits_{i \neq k}^n \sum\limits_{j=1}^n a_i a_k a_j^2 + \sum\limits_{i \neq k}^n \sum\limits_{j \neq l}^n a_i a_k a_j a_l \end{align}
but I don't know is it correct or how to prove it.