How would you write this in Sigma notation I stumbled upon this expression:$$p_nq_mx^{m+n}+(p_{n-1}q_m+p_nq_{m-1})x^{m+n-1}+(p_{n-2}q_m+p_{n-1}q_{m-1}+p_nq_{m-2})x^{m+n-2}+\ldots+p_0q_0\tag1$$
And I'm wondering if there is an easier way to represent $(1)$ using the sum notation Sigma: $\sum$.

$$\sum_{i=0}^{n}\sum_{j=0}^{m} p_iq_jx^{i+j}$$
