TITLE: Formal power series coefficient multiplication
QUESTION [7 upvotes]: Given that I have two formal power series:
$$ A(x) = \sum_{k \ge 0} a_k x^k $$
$$ B(x) = \sum_{k \ge 0} b_k x^k $$
The Cauchy Product gives a series
$$ C(x) = \sum_{k \ge 0} c_k x^k $$
$$ c_k  = \sum_{n=0}^k a_n b_{k-n} $$
Which comes from taking the product of the two series $C(x)=A(x)B(x)$. What then, in terms of $A(x)$ and $B(x)$, is this series?
$$Y(x) = \sum_{k \ge 0} a_k b_k x^k $$

REPLY [2 votes]: For more on Hadamard Products or Hadamard Multiplication Theorem look for papers by Louis R. Bragg in American Mathematical Monthly, Jan. 1999, pp 36-42 or SIAM J. Math. Anal., Vol. 17, 1986, pp 220-230 for starters.