Let $R$ be a local ring , $\mathfrak{m}$ the maximal ideal, $q$ is $\mathfrak m$-primary . Then we can prove that there exists a polynomial $F_q(t)\in \mathbb{Q}(t)$ such that $F_q(n)=\mathcal{l}(R/\mathfrak q^n)$ for $n>>0$ . Let $d_q(R)$ be the degree of $F_q(t)$ . Then we can prove that the value of $d_q(R)$ don't depend on the choice of $q$ .
Now we want to show that $d(R/(a))\leqq d(R)-1$ where $a$ is not a zero-divisor. Then the degree of $\mathcal l(\bar R/\bar {\mathfrak m}^n)$ where $\bar R=R/(a)$ and $\bar {\mathfrak m}=\mathfrak m/(a)$ is less than $d(R)-1$ .
My question is how to get the conclusion we want by this?