24 (copy) (copy) (copy) (copy) (copy) (copy) (copy) (copy) (copy) (copy) (copy) (copy)

$ \left ( \sum_{n = 1}^{\infty} a_n \right )\left ( \sum_{n = 1}^{\infty} b_n \right ) = \sum_{n = 2}^{\infty} \sum_{k = 1}^{n-1} a_k b_{n-k} = a_1b_1 + (a_2b_1 + a_1b_2) + \dots

$ \left ( \sum_{n = 1}^{\infty} a_n \right )\left ( \sum_{n = 1}^{\infty} b_n \right ) = \sum_{n = 2}^{\infty} \sum_{k = 1}^{n-1} a_k b_{n-k} = a_1b_1 + (a_2b_1 + a_1b_2) + \dots

$ \left ( \sum_{n = 1}^{\infty} a_n \right )\left ( \sum_{n = 1}^{\infty} b_n \right ) = \sum_{n = 2}^{\infty} \sum_{k = 1}^{n-1} a_k b_{n-k} = a_1b_1 + (a_2b_1 + a_1b_2) + \dots

$ \left ( \sum_{n = 1}^{\infty} a_n \right )\left ( \sum_{n = 1}^{\infty} b_n \right ) = \sum_{n = 2}^{\infty} \sum_{k = 1}^{n-1} a_k b_{n-k} = a_1b_1 + (a_2b_1 + a_1b_2) + \dots

$ \left ( \sum_{n = 1}^{\infty} a_n \right )\left ( \sum_{n = 1}^{\infty} b_n \right ) = \sum_{n = 2}^{\infty} \sum_{k = 1}^{n-1} a_k b_{n-k} = a_1b_1 + (a_2b_1 + a_1b_2) + \dots