A219907 Numbers of the form 4x^2+2xy+7y^2, x, y >= 0.

0, 4, 7, 13, 16, 27, 28, 36, 49, 52, 63, 64, 73, 76, 79, 91, 100, 108, 112, 117, 124, 144, 148, 151, 163, 172, 175, 189, 193, 196, 208, 211, 217, 241, 243, 252, 256, 268, 279, 292, 301, 304, 316, 324, 325, 343, 349, 361, 364, 367, 379, 387

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

Cf. A219908.

Sequence in context: A309794 A048297 A061201 * A310818 A310819 A310820

Adjacent sequences: A219904 A219905 A219906 * A219908 A219909 A219910

Keyword

nonn

Author

N. J. A. Sloane, Dec 14 2012

Status

approved