Exchange of product and sum
All products added together are multiplied by the sum
https://gyazo.com/9906e3c1196aa99b1fb7875ceed248b0
$ \sum_{i,j} a_i \times b_j = (\sum_i a_i) \times (\sum_j b_j)
It looks like a special law, but it just uses a lot of distributive law.
Distribution law: $ a \times b + a \times c = a \times (b + c)
More use formal power series (corresponding to an infinite sequence of numbers) for infinite sums
$ \sum_i^\infty x^{ai} \times \sum_j^\infty x^{bj} = \sum_i^\infty\sum_j^\infty x^{ai+bj}
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