# Convolution of one signal with an evenly spaced signal

I do not specialize in signal processing so I wonder if there is any references to the following procedure.

Let $[n]=\{0,1,\ldots,n\}$.

Consider the function $f:[n] \to \mathbb{R}$ and $g:[kr] \to \mathbb{R}$, such that $\operatorname{supp} g = \{0,r,2r,3r,\ldots,kr\}$. Assume for simplicity, $n/r$ is a integer.

We are interested in computing the linear convolution $f*g$.

If $r$ is large, one way to do the computation is partition $f$ into $r$ functions $f_0,\ldots,f_{r-1}$, such that $f_i(t) = f(tr+i)$, and $f_i:[n/r]\to \mathbb{R}$.

We also define $\bar{g}:[k]\to \mathbb{R}$ as $\bar{g}(t) = g(rt)$.

Find $f_i*\bar{g}$ to each $i$, then we can read off $(f*g)(tr+i) = (f_i*\bar{g})(t)$.

This make sure we apply $r$ convolutions on sequence of length $n/r$ and $k$. Instead of $1$ convolution on sequence of length $n$ and $kr$.