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A028955 Numbers represented by quadratic form with Gram matrix [ 4, 1; 1, 4 ] (divided by 2). 4
0, 2, 3, 5, 8, 12, 17, 18, 20, 23, 27, 30, 32, 38, 45, 47, 48, 50, 53, 57, 62, 68, 72, 75, 80, 83, 92, 93, 95, 98, 102, 107, 108, 113, 120, 122, 125, 128, 137, 138, 147, 152, 153, 155, 158, 162, 167, 170, 173, 180, 183, 188, 192, 197, 200, 207, 212, 218, 227, 228 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers of the form 2*x^2 + x*y + 2*y^2, of discriminant -15. - N. J. A. Sloane, Jun 01 2014

8*a(n) is of the form z^2 + 15*y^2, where z = 4*x + y. [Bruno Berselli, Jul 12 2014]

LINKS

Table of n, a(n) for n=1..60.

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

FORMULA

a(x, y) = (4x^2 + 2xy + 4y^2)/2; x, y any integer.

EXAMPLE

32 is in the sequence because it can be written in the form 2*2^2+2*3+2*3^2, and hence 8*32 = 11^2+15*3^2.

CROSSREFS

Cf. A028927. For primes see A106859.

Sequence in context: A238548 A004170 A247116 * A246321 A104664 A022856

Adjacent sequences:  A028952 A028953 A028954 * A028956 A028957 A028958

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane.

EXTENSIONS

More terms from Larry Reeves (larryr(AT)acm.org), Mar 29 2000

STATUS

approved

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Last modified January 21 13:55 EST 2020. Contains 331113 sequences. (Running on oeis4.)