A135658 Nonprimes of the form 4x^2-4xy+7y^2.

0, 4, 15, 16, 24, 28, 36, 40, 55, 60, 63, 64, 87, 88, 96, 100, 112, 124, 132, 135, 144, 159, 160, 168, 175, 196, 216, 220, 231, 232, 240, 252, 256, 279, 280, 292, 295, 303, 316, 319, 324, 343, 348, 352, 360, 375, 384, 388, 400, 412, 415, 420, 424, 447, 448, 472, 484, 495, 496, 508, 511, 519, 528

Offset

1,2

Comments

Because 4x^2-4*x*y+7*y^2 = (2*x-y)^2+6*y^2, this is a subsequence of A002481. - R. J. Mathar, Jan 18 2021

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

N:= 1000: # for terms <= N
R:= {}:
for x from 0 while 24*x^2/7 < N do
  for y from 0 do
    v:= 4*x^2 - 4*x*y + 7*y^2;
    if v <= N then if not isprime(v) then R:= R union {v} fi
    elif y > 2*x/7 then break
fi od od:
sort(convert(R, list)); # Robert Israel, Sep 30 2025

Crossrefs

Cf. A002481, A107006, A124477, A135657.

Sequence in context: A260577 A152442 A161769 * A228589 A066862 A228590

Adjacent sequences: A135655 A135656 A135657 * A135659 A135660 A135661

Keyword

nonn

Author

Artur Jasinski, Nov 25 2007

Status

approved