Okay, here we have this:
Considering the provided secuence: 81,85,89,93,97. Let's calculate the difference between each term to identify if it is constant or not:
85-81=4
89-85=4
93-89=4
97-93=4
Here we can see that the difference is a constant, so this mean that it's an Arithmetic sequence, now, let's replace in the formula:
[tex]a_n=a_1+(n-1)d[/tex]Replacing:
[tex]a_n=81+(n-1)4[/tex]And, turning this in a recursive function:
[tex]g(n)=\begin{cases}81\text{ if n=}1 \\ g(n-1)+4\text{ if }n>1\end{cases}[/tex]