$L^1(\mathbb R^n)$, $L^1(\mathbb R_+)$, $C^0_c(\mathbb R_+)$, $C^\infty_c(\mathbb R_+)$ are algebras of convolution.
Question 1: is there a classification of subalgebras of convolution of $L^1(\mathbb R^n)$?
Question 2: is there a classification of subalgebras of convolution of $\mathscr E'(\mathbb R^n)$ (distributions on $\mathbb R^n $ with compact support)?