The On-Line Encyclopedia of Integer Sequences
I was trying to answer this question – how many labeled rooted trees with n nodes are there?
A quick search didn’t find the answer (I’m sure a more detailed search would have). Then I had this idea – find the answer for small values of n, and look in the OEIS. I typed 1,2,9,64 in the search and quickly found the answer (which is n^(n-1) for those interested). I thought about it for a couple of minutes but still hadn’t come up with an answer as to why this is true.
