# How many structurally different trees can be formed with n nodes?

Is there any exact formula for finding number of structurally different unlabeled trees can be formed with $n$ nodes? I searched a lot and I found some approximation formulas, e.g. here.

• Have you tried counting them? In addition to Juho's question, are they ordered/plane? If there's an answer, it's probably in Flajolet/Sedgewick Analytic Combinatorics. – Raphael Sep 25 '14 at 15:16
• @Raphael As Juho noted, Wikipedia states that no closed formula is known. – Gilles Sep 25 '14 at 17:39