# How one can simplify this product of sums

My question is simple: How one can simplify this product of sums:

$$S=\left(\sum_{k=1}^{n}a_{k}\right) \left(\sum_{k=1}^{p}b_{k}\right) \left(\sum_{k=1}^{q}c_{k}\right)$$

where $a_{k},b_{k},c_{k}$ are real sequences and $n<p<q$.

• What do you mean by simplify? I think that is as compact as things can get without knowing a,b,c. If you want to expand them you can assign different indices, but I wouldn't call that simpler! – Kitter Catter Nov 3 '16 at 17:51
• Can you find an alternative expression simplier than $a(b_1+b_2)(c_1+c_2+c_3)$? – Ng Chung Tak Nov 3 '16 at 17:53

Don't you have $$S=\left(\sum_{k=1}^{n}a_{k}\right) \left(\sum_{k=1}^{p}b_{k}\right) \left(\sum_{k=1}^{q}c_{k}\right) = \sum_{i=1}^n \sum_{j=1}^p \sum_{k=1}^q a_i b_j c_k?$$