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A300360 Number of ways to write n^2 as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers and z <= w such that x or y is a power of 2 (including 1) and x + 63*y = 2^(2k+1) for some k = 0,1,2,.... 6
0, 1, 2, 1, 1, 1, 2, 1, 2, 1, 2, 3, 1, 1, 3, 1, 3, 4, 2, 2, 4, 1, 2, 1, 1, 2, 4, 3, 1, 1, 2, 1, 6, 2, 2, 2, 5, 1, 4, 1, 2, 6, 3, 3, 3, 1, 2, 3, 4, 3, 3, 2, 4, 2, 2, 1, 7, 3, 1, 4, 1, 2, 8, 1, 3, 7, 3, 4, 6, 3, 4, 4, 6, 4, 3, 2, 4, 3, 1, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Conjecture: a(n) > 0 for all n > 1, and a(n) = 1 only for n = 5, 13, 25, 29, 59, 61, 79, 91, 95, 101, 103, 1315, 2^k (k = 1,2,3,...), 2^(2k+1)*m (k = 0,1,2,... and m = 3, 5, 7, 11, 15, 19, 23).

Note the difference between the current sequence and A300356.

In the comments of A300219, the author conjectured that a positive square n^2 can be written as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers such that both x and x + 3*y are powers of 4 unless n has the form 4^k*81503 with k a nonnegative integer. Since 81503^2 = 208^2 + 16^2 + 51167^2 + 63440^2 with 16 = 4^2 and 208 + 3*16 = 4^4, this implies that any positive square can be written as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers such that x or y is a power of 4 and x + 3y is also a power of 4. We also conjecture that for any positive integer n not of the form 4^k*m (k =0,1,... and m = 2, 7) we can write n^2 as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers such that x or y is a power of 4 and x + 2*y is also a power of 4.

LINKS

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

Zhi-Wei Sun, Refining Lagrange's four-square theorem, J. Number Theory 175(2017), 167-190.

Zhi-Wei Sun, Restricted sums of four squares, arXiv:1701.05868 [math.NT], 2017-2018.

EXAMPLE

a(38) = 1 since 38^2 = 2^2 + 0^2 + 12^2 + 36^2 with 2 = 2^1 and 2 + 63*0 = 2^1.

a(86) = 2 since 86 = 65^2 + 1^2 + 19^2 + 53^2 = 65^2 + 1^2 + 31^2 + 47^2 with 1 = 2^0 and 65 + 63*1 = 2^7.

a(535) = 3 since 535^2 = 2^2 + 130^2 + 64^2 + 515^2 = 2^2 + 130^2 + 139^2 + 500^2 = 8^2 + 520^2 + 40^2 + 119^2 with 2 = 2^1, 8 = 2^3, 2 + 63*130 = 2^13 and 8 + 63*520 = 2^15.

a(1315) = 1 since 1315^2 = 512^2 + 512^2 + 61^2 + 1096^2 with 512 = 2^9 and 512 + 63*512 = 2^15.

MATHEMATICA

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

Pow[a_, n_]:=Pow[a, n]=IntegerQ[Log[a, n]];

tab={}; Do[r=0; Do[If[SQ[n^2-x^2-y^2-z^2]&&(Pow[2, x]||Pow[2, y])&&Pow[4, (x+63y)/2], r=r+1], {x, 0, n}, {y, 0, Sqrt[n^2-x^2]}, {z, 0, Sqrt[(n^2-x^2-y^2)/2]}]; tab=Append[tab, r], {n, 1, 80}]; Print[tab]

CROSSREFS

Cf. A000079, A000118, A000290, A271518, A279612, A281976, A299924, A300219, A300356, A300362.

Sequence in context: A330754 A330753 A082068 * A300396 A300356 A082069

Adjacent sequences:  A300357 A300358 A300359 * A300361 A300362 A300363

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Mar 03 2018

STATUS

approved

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Last modified July 15 00:36 EDT 2020. Contains 335762 sequences. (Running on oeis4.)