The book entitled "Assayer" published in 1623 was a controversial text about the theory of comets between Galileo and his opponent Horatio Grassi, the Jesuit mathematician. Galileo was an author of the original but false optical theory of comets while Grassi proposed a supra-lunar origin of comets supporting Tycho's opinion on comets and their motion. Unfortunately, Grassi's theory was based on a misunderstanding of how the telescope actually works., Vladimír Štefl., and Obsahuje bibliografii
Let $FG$ be a group algebra of a group $G$ over a field $F$ and ${\mathcal U}(FG)$ the unit group of $FG$. It is a classical question to determine the structure of the unit group of the group algebra of a finite group over a finite field. In this article, the structure of the unit group of the group algebra of the non-abelian group $G$ with order $21$ over any finite field of characteristic $3$ is established. We also characterize the structure of the unit group of $FA_4$ over any finite field of characteristic $3$ and the structure of the unit group of $FQ_{12}$ over any finite field of characteristic $2$, where $Q_{12}=\langle x, y; x^6=1, y^2=x^3, x^y=x^{-1} \rangle $.