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A219907
Numbers of the form 4x^2+2xy+7y^2, x, y >= 0.
3
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
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
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Dec 14 2012
STATUS
approved