## Monday, February 3, 2014

### Matrix multiplication as a convolution

1. The product of two power series/polynomials is \begin{align*} & (a_0 + a_1 x + a_2 x^2 + \cdots) (b_0 + b_1 x + b_2 x^2 + \cdots) \\ = & c_0 + c_1 x + c_2 x^2 + \cdots \end{align*} The coefficients given by $$c_n = \sum_{k=0}^n a_k b_{n-k}$$ is sometimes called the Cauchy product. This is a convolution.

2. Let $G$ be a finite group and $C[G]$ be the group algebra with complex coefficients. Let $$f = \sum_x f(x) x, g = \sum_y g(y) y$$ be two elements in $C[G]$. The product is $$f g = \sum_z ( \sum_{x y = z} f(x) g(y) ) z.$$ When $f$ and $g$ are treated as functions $f, g : G \to C$, $$(f g) (z) = \sum_{x y = z} f(x) g(y)$$ is a convolution. In fact, $C[G]$ is an example of convolution algebra $L^1 (G)$.

3. Let $J = \{1, ... ,n \}$ and $G' = J \times J = \{ ( i, j ) : 1≤ i, j ≤n \}$. $G'$ is a groupoid: not every pair of elements in $G'$ can be composed, $(a, b)$ can only be composed with $(c,d)$ when $b=c$. In such case, $(i, j) (j, k) = (i, k).$
Let us consider the groupoid convolution algebra $C[G']$ as above. The convolution in this case is then $$(f g) (i,k) = \sum_{ (i, j) (j, k) = (i, k) } f(i, j) g(j, k).$$ If we rewrite this in another format, it should look more familiar: $$(AB)_{ik} = \sum_{i=1}^n A_{ij} B_{jk}.$$ This is matrix multiplication.