On Christmas Day I alluded to the fact that there are 364 gifts in the song “The Twelve Days of Christmas”. Is there a way to prove this that doesn’t require adding everything up?
As a reminder, on day k of Christmas (k = 1, 2, …, 12) the singer receives 1 of gift 1, 2 of gift 2, …, k of gift k. Christmas has 12 days. (Gift 1 is “a partridge in a pear tree”, gift 2 is “turtle doves”, and so on up to gift 12 which is “drummers drumming”, but this is irrelevant.)
So on day k there are total of gifts; this is . The total number of gifts received is therefore
and by the hockey-stick identity (sometimes also called the Christmas stocking identity) this is . The identity can be proven by induction, but I prefer a combinatorial proof. Consider the subsets of of size 3 and group them according to their largest element. Then there are sets whose largest element is , for each of .
This suggests another identity – what if we group according to the middle element of the subset instead? For example, there are 5 × 8 = 40 3-subsets of  whose middle element is 6; each one has one element chosen from 1, 2, …, 5 and one element chosen from 7, 8, …, 14. More generally there are k(13-k) 3-subsets of  with middle element k. Thus we have
In terms of the song, this is actually a natural way to count. is the number of gifts of type k, since such gifts get given on the last days – there are 12 total partridges in pear trees, 2 × 11 = 22 total turtle doves, 3 × 10 = 30 total calling birds, and so on until we get back down to 12 drummers drumming. (The most frequent gifts? 42 swans and 42 geese. Maybe that was the question.)