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A275676 Number of ordered ways to write n as 4^k*(1+5*x^2+y^2) + z^2, where k,x,y,z are nonnegative integers with x <= y. 7
1, 2, 1, 1, 3, 2, 1, 3, 2, 3, 4, 1, 1, 3, 1, 3, 4, 2, 3, 3, 3, 1, 2, 3, 2, 7, 2, 1, 4, 3, 4, 5, 3, 2, 4, 2, 4, 4, 1, 5, 8, 3, 2, 4, 1, 7, 3, 1, 2, 4, 5, 1, 5, 2, 4, 7, 3, 3, 5, 1, 3, 5, 1, 6, 6, 7, 2, 4, 5, 2, 9, 3, 4, 6, 3, 3, 2, 2, 4, 7 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjecture: (i) a(n) > 0 for all n > 0.

(ii) Any positive integer can be written as 4^k*(1+5*x^2+y^2) + z^2, where k,x,y,z are nonnegative integers with y <= z.

See also A275656, A275675 and A275678 for similar conjectures.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..10000

Zhi-Wei Sun, Refining Lagrange's four-square theorem, arXiv:1604.06723 [math.GM], 2016.

EXAMPLE

a(4) = 1 since 4 = 4*(1+5*0^2+0^2) + 0^2 with 0 = 0.

a(259) = 1 since 259 = 4^0*(1+5*4^2+13^2) + 3^2 with 4 < 13.

a(333) = 1 since 333 = 4*(1+5*3^2+5^2) + 7^2 with 3 < 5.

a(621) = 1 since 621 = 4*(1+5*0^2+8^2) + 19^2 with 0 < 8.

a(717) = 1 since 717 = 4*(1+5*3^2+11^2) + 7^2 with 3 < 11.

a(1581) = 1 since 1581 = 4*(1+5*1^2+3^2) + 39^2 with 1 < 3.

a(2541) = 1 since 2541 = 4*(1+5*3^2+13^2) + 41^2 with 3 < 13.

MATHEMATICA

SQ[n_]:=SQ[n]=IntegerQ[Sqrt[n]]

Do[r=0; Do[If[SQ[n-4^k*(1+5x^2+y^2)], r=r+1], {k, 0, Log[4, n]}, {x, 0, Sqrt[(n/4^k-1)/6]}, {y, x, Sqrt[n/4^k-1-5x^2]}]; Print[n, " ", r]; Continue, {n, 1, 80}]

CROSSREFS

Cf. A000118, A000290, A271518, A275648, A275656, A275675, A275678

Sequence in context: A327523 A190770 A292149 * A025831 A184751 A296150

Adjacent sequences:  A275673 A275674 A275675 * A275677 A275678 A275679

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Aug 04 2016

STATUS

approved

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Last modified August 1 14:14 EDT 2021. Contains 346391 sequences. (Running on oeis4.)