A301891 - OEIS

%I #8 Mar 29 2018 10:08:16

%S 2,1,1,2,3,1,2,1,3,1,2,1,6,3,8,2,5,4,6,3,4,3,6,1,3,3,7,2,8,9,8,1,15,5,

%T 8,3,11,1,5,1,4,4,2,2,10,7,17,1,18,11,14,6,16,6,17,3,21,10,16,8,19,8,

%U 30,2,15,9,18,5,28,5,27,4,13,11,24,6,28,17,20,3

%N Number of ways to write 2*n^2 as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers such that x + 3*y + 5*z + 15*w is a power of 4.

%C Conjecture 1: a(n) > 0 for all n > 0, and a(n) = 1 only for n = 4^k*m with k = 0,1,2,... and m = 2, 3, 6, 10, 38.

%C Conjecture 2: For any positive integer n, we can write 2*n^2 as x^2 + y^2 + z^2 + w^2 with x,y,z,w nonnegative integers such that x + 3*y + 5*z + 15*w is twice a power of 4.

%H Zhi-Wei Sun, <a href="/A301891/b301891.txt">Table of n, a(n) for n = 1..225</a>

%H Zhi-Wei Sun, <a href="http://dx.doi.org/10.1016/j.jnt.2016.11.008">Refining Lagrange's four-square theorem</a>, J. Number Theory 175(2017), 167-190.

%H Zhi-Wei Sun, <a href="http://arxiv.org/abs/1701.05868">Restricted sums of four squares</a>, arXiv:1701.05868 [math.NT], 2017-2018.

%e a(1) = 2 since 2*1^2 = 1^2 + 1^2 + 0^2 + 0^2 with 1 + 3*1 + 5*0 + 15*0 = 4, and 2*1^2 = 1^2 + 0^2 + 0^2 + 1^2 with 1 + 3*0 + 5*0 + 15*1 = 4^2.

%e a(2) = 1 since 2*2^2 = 0^2 + 2^2 + 2^2 + 0^2 with 0 + 3*2 + 5*2 + 15*0 = 4^2.

%e a(3) = 1 since 2*3^2 = 1^2 + 1^2 + 0^2 + 4^2 with 1 + 3*1 + 5*0 + 15*4 = 4^3.

%e a(6) = 1 since 2*6^2 = 0^2 + 8^2 + 2^2 + 2^2 with 0 + 3*8 + 5*2 + 15*2 = 4^3.

%e a(10) = 1 since 2*10^2 = 10^2 + 8^2 + 6^2 + 0^2 with 10 + 3*8 + 5*6 + 15*0 = 4^3.

%e a(38) = 1 since 2*38^2 = 34^2 + 34^2 + 24^2 + 0^2 with 34 + 3*34 + 5*24 + 15*0 = 4^4.

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

%t Pow[n_]:=Pow[n]=IntegerQ[Log[4,n]]

%t tab={};Do[r=0;Do[If[SQ[2n^2-x^2-y^2-z^2]&&Pow[x+3y+5z+15*Sqrt[2n^2-x^2-y^2-z^2]],r=r+1],{x,0,Sqrt[2]n},{y,0,Sqrt[2n^2-x^2]},{z,0,Sqrt[2n^2-x^2-y^2]}];tab=Append[tab,r],{n,1,80}];Print[tab]

%Y Cf. A000290, A000302, A271518, A281976, A299924, A299537, A299794, A300219, A300362, A300396, A300510, A301376, A301391, A301452, A301471, A301579, A301640.

%K nonn

%O 1,1

%A _Zhi-Wei Sun_, Mar 28 2018