Probability of zero-occurrences of a substring within a string

Question

Let's assume that we have a string S of length n < ∞ which is consisted of an alphabet of 20 different letters. Each letter does not occur with the same frequency. Let's also assume a substring s of length k < n (k-mer).

So, the question is what's the probability of the substring s to not occur within the string S?

Answer 1

The results of the formula that you see in the post that I have referred to!!

\begin{align*} \begin{array}{|l|c|}\hline T & P \\\hline 1 & 0.278730925452149 \\ 2 & 0.059428898295207 \\ 3 & 0.008431600808656 \\ 4 & 0.000895516555326\\ \hline \end{array} \end{align*}