I don't remember offhand no. But aparently it's complicated...it would have to be to generate the following sequence, starting with n=1:
1,1,1,1,2,3,5,9,18,35,75,159,355,802,1858,4347,10359,24894, 60523,148284,366319,910726,2278658,5731580,14490245, 36797588,93839412,240215803,617105614,1590507121,4111846763, 10660307791,27711253769
Maybe someone with a huge math background could get us the formula from those numbers?
![Huh ???](https://www.chemicalforums.com/Smileys/classic/huh.gif)
ALSO for reference this is sequence
A000602 in
Online Encyclopedia of Integer Sequences.
P.S. All that info plus more can be found in WikiPedia