Some remarks on Pr\"{u}fer $\star $--multiplication domains and class groups

Let $D$ be an integral domain with quotient field $K$ and let $X$ be an indeterminate over $D$. Also, let $\boldsymbol{\mathcal{T}}:=\{T_{\lambda}\mid \lambda \in \Lambda \}$ be a defining family of quotient rings of $D$ and suppose that $\ast$ is a finite type star operation on $D$ induced by $\boldsymbol{\mathcal{T}}$. We show that $D$ is a P$\ast$MD (resp., P$v$MD) if and only if $(\co_D(fg))^{\ast}=(\co_D(f)\co_D(g))^{\ast}$ (resp., $(\co_D(fg))^{w}=(\co_D(f)\co_D(g))^{w}$) for all $0 \ne f,g \in K[X]$. A more general version of this result is given in the semistar operation setting. We give a method for recognizing P$v$MD's which are not P$\ast$MD's for a certain finite type star operation $\ast$. We study domains $D$ for which the $\ast$--class group $\Cl^{\ast}(D)$ equals the $t$--class group $\Cl^{t}(D)$ for any finite type star operation $\ast$, and we indicate examples of P$v$MD's $D$ such that $\Cl^{\ast}(D)\subsetneq \Cl^{t}(D)$. We also compute $\Cl^v(D)$ for certain valuation domains $D$.