Presumably this is a well-known identity. Does it have a name?pic.twitter.com/tsMBGLZGAk
You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
I know two proofs. One is to show that both expressions satisfy the recurrence f(n, k+1) = \sum_1^n i f(i, k). The other is to consider the probability that, rolling a fair n-sided die (n+k) times, you see all n sides of the die; and compute this probability in two different ways
If your proof is neither of these, I would love to see it.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.