  Let $\sigma_n=\lfloor1+n\cdot\log_23\rfloor$. For the Collatz 3x + 1 function
exists for each $n\in\mathbb{N}$ a set of different residue classes
$(\text{mod}\ 2^{\sigma_n})$ of starting numbers $s$ with finite stopping time
$\sigma(s)=\sigma_n$. Let $z_n$ be the number of these residue classes for each
$n\geq0$ as listed in the OEIS as A100982. It is conjectured that for each
$n\geq4$ the value of $z_n$ is given by the formula \begin{align*}
z_n=\frac{(m+n-2)!}{m!\cdot(n-2)!}-\sum_{i=2}^{n-1}\binom{\big\lfloor\frac{3(n-i)+\delta}{2}\big\rfloor}{n-i}\cdot
z_i, \end{align*} where $m=\big\lfloor(n-1)\cdot\log_23\big\rfloor-(n-1)$ and
$\delta\in\mathbb{Z}$ assumes different values within the sum at intervals of 5
or 6 terms. This allows us to create an iterative algorithm which generates
$z_n$ for each $n>6$.
