4 hours later…
2 hours later…
If you have a look at the linked questions to the first one, there is also a fourth question about the same sum. But since it specifically asks for combinatorial proof, I would not consider it to be a duplicate.
Since one of those questions was posted by me, it is probably better if I let other users decide whether they should be closed and in which direction.
« first day (1362 days earlier) ← previous day