Good question. I just presented it here the way it was given to me, as a couple initial terms and some rules on how it evolves, because I find that to be the e

Good question. I just presented it here the way it was given to me, as a couple initial terms and some rules on how it evolves, because I find that to be the easiest way to remember it. The actual formula looks much less friendly (because it's tricky to write "the product of the first n odd integers"), but it's a good exercise for those who are inclined.