I'm trying to find a closed form expression for the following sequence:
Where $n$ and $d$ are both positive integers.
An important note!: I'm actually not even interested in an exact solution. Any closed form that is a 'reasonably' close approximation is fine!
Now I have no reason to assume a close form even exists, and the expression looks quite ugly, so I would even go as far as to say that I highly doubt a closed form exists, but maybe someone can take a look and tell why it does/does not exist.
Edit: an other way to write the sequence is as:
Maybe someone will recognize this or know how to deal with this.