OFFSET
1,2
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
W. C. Jagy, Problem 11539, Amer. Math. Monthly, 119 (2012), 884-885.
PROG
(Haskell)
import Data.Set (deleteFindMin, empty, fromList, union)
import qualified Data.Set as Set (null)
a219907 n = a219907_list !! (n-1)
a219907_list = f 0 empty where
f z s | Set.null s || z' <= m = f (z + 1) (s `union` (fromList ws))
| otherwise = m : f z s'
where (m, s') = deleteFindMin s
ws = map (h z) [0..z] ++ map (flip h z) [0..z-1]
h i j = 4 * i ^ 2 + 2 * i * j + 7 * j ^ 2
z' = h z 0
-- Reinhard Zumkeller, Dec 15 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Dec 14 2012
STATUS
approved