This is something I haven't seen online yet, indicator functions with values in a finite field. Probably for a good reason, but I would like to know why, and if there are still things that can be said. For instance what can we say of the relationship (if any) between the support of the convolution of two indicator functions with values in a finite field vs the addition of the sets they indicate.
More precisely: Let $F$ be a finite field and let $A, B$ be sets with additive cyclic structure, say $A, B \subseteq \mathbb{Z}_N$ with $N$ coprime to the characteristic of $F$. Define the "characteristic functions" $1_A, 1_B : \mathbb{Z}_N \to F$ by $1_A(x) = 1$ if $x \in A$, and $1_A(x) = 0$ otherwise. Similarly for $1_B$. Is there any relationship/proposition we can infer between the set $A + B$ and the support of the (cyclic) convolution $$ 1_A \ast 1_B(k) = \sum_{j \in \mathbb{Z}_N} 1_A(j) 1_B(k-j)? $$ For instance, if $1_A \ast 1_B(k) = 0$ for all $k \in \mathbb{Z}_N$, would this say anything at all about $A + B$?
Note in the case of the usual characteristic functions with values in $\mathbb{R}$ we have the nice property that $A + B$ is precisely the support of $1_A \ast 1_B$. So I wondered whether we can at least get something (although probably not quite this I imagine) when the values of the characteristic functions are either the additive or multiplicative identity in a finite field. Thanks!
Remark: Recall that the convolution here is taken modulo $p$, where $p$ is the characteristic of the field. Moreover note that the support of $1_A \ast 1_B$ is contained in $A + B$, since, if the convolution is non-zero modulo $p$, it is non-zero in $\mathbb{R}$ and so the support lies in $A + B$ by the fact mentioned above. This however does not say anything on whether or not the support is empty modulo $p$...