more Π obscurantism

Here's an illustration of an approach to Π pointed out by Kevin Brown. Define f(n) as the nearest greater or equal multiple of n-1, then of n-2, etc (yielding OEIS sequence 2491). Then, inverting a result found by Duane Broline and Daniel Loeb, Π = n² / f(n).

But as you can see from the comment, the series converges very slowly!

	main = print $ e**2 / f e e 1
	where
	e = 900000 -- yields 3.1416003...
	f n 1 _ = n
	f n k l = f n' (k - 1) $ n' / k
	where
	n' = head $ dropWhile (< n) $ map (* k) [l..]

view raw kevin_brown_PI.hs hosted with ❤ by GitHub