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A242663 Nonnegative integers of the form x^2 + 4*x*y - 4*y^2. 4
0, 1, 4, 8, 9, 16, 17, 25, 28, 32, 36, 41, 49, 56, 64, 68, 72, 73, 81, 89, 92, 97, 100, 112, 113, 121, 124, 128, 136, 137, 144, 153, 161, 164, 169, 184, 188, 193, 196, 200, 217, 224, 225, 233, 241, 248, 252, 256, 257, 272, 281, 284, 288, 289, 292 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Discriminant 32.

Also numbers representable as x^2 + 6*x*y + y^2 with 0 <= x <= y. - Gheorghe Coserea, Jul 29 2018

Also numbers of the form x^2 - 8*y^2. - Jianing Song, Jul 31 2018

LINKS

Gheorghe Coserea, Table of n, a(n) for n = 1..100001

N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)

MATHEMATICA

Reap[For[n = 0, n <= 300, n++, If[Reduce[ x^2 + 4*x*y - 4*y^2 == n, {x, y}, Integers] =!= False, Sow[n]]]][[2, 1]]

PROG

(PARI)

seq(M, k=6) = {

setintersect([1..M], setbinop((x, y)->x^2 + k*x*y + y^2, [0..1+sqrtint(M)]));

};

concat(0, seq(292)) \\ Gheorghe Coserea, Jul 31 2018

CROSSREFS

Cf. A031363.

Primes in this sequence = A007519.

Sequence in context: A237882 A153034 A106840 * A160053 A140612 A225353

Adjacent sequences:  A242660 A242661 A242662 * A242664 A242665 A242666

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, May 31 2014

STATUS

approved

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Last modified May 17 19:26 EDT 2021. Contains 343988 sequences. (Running on oeis4.)