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A256106
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Number of ways to write n as w + x + 2*y + 4*z, where w,x,y,z are hexagonal numbers with w <= x.
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2
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1, 1, 2, 1, 2, 1, 3, 2, 2, 1, 1, 1, 3, 2, 2, 1, 3, 2, 4, 2, 1, 2, 2, 2, 2, 2, 2, 3, 4, 1, 4, 4, 5, 3, 4, 2, 6, 3, 3, 1, 4, 3, 4, 2, 1, 5, 5, 3, 2, 3, 3, 4, 3, 2, 3, 5, 4, 4, 5, 2, 7, 4, 8, 4, 5, 1, 6, 5, 5, 5, 4, 3, 9, 4, 3, 6, 5, 5, 4, 5, 3, 6, 5, 4, 5, 4, 4, 4, 5, 3, 10, 5, 8, 4, 7, 3, 11, 8, 3, 4, 5
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OFFSET
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0,3
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COMMENTS
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Conjecture: (i) a(n) > 0 for all n.
(ii) Any nonnegative integer n can be written as w + b*x + c*y + d*z with w,x,y,z pentagonal numbers, provided that (b,c,d) is one of the following 15 triples: (1,1,2), (1,2,2), (1,2,3), (1,2,4), (1,2,5), (1,2,6), (1,3,6), (2,2,4), (2,2,6), (2,3,4), (2,3,5), (2,3,7), (2,4,6), (2,4,7), (2,4,8).
I have shown the following related result: For m > 4 and 0 < a <= b <= c <= d, if every nonnegative integer can be written as a*w + b*x + c*y + d*z with w,x,y,z m-gonal numbers, then either m = 6 and (a,b,c,d) = (1,1,2,4), or m = 5 and a = 1 and (b,c,d) is among the 15 triples listed in part (ii) of the conjecture.
In the preprint arXiv:1608.02022, Xiang-Zi Meng and Zhi-Wei Sun confirmed part (i) of the conjecture, and they also proved that for each triple (b,c,d) = (1,2,2),(1,2,4) any natural number can be written as w + b*x + c*y + d*z with w,x,y,z pentagonal numbers. Zhi-Wei Sun, Aug 09 2016
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LINKS
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EXAMPLE
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a(65) = 1 since 65 = 1*(2*1-1) + 4*(2*4-1) + 2*2*(2*2-1) + 4*2*(2*2-1) = 1 + 28 + 2*6 + 4*6 with 1,28,6,6 hexagonal numbers.
a(104) = 1 since 104 = 1*(2*1-1) + 7*(2*7-1) + 2*2*(2*2-1) + 4*0*(2*0-1) = 1 + 91 + 2*6 + 4*0 with 1,91,6,0 hexagonal numbers.
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MATHEMATICA
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H[n_]:=IntegerQ[Sqrt[8n+1]]&&(n==0||Mod[Sqrt[8n+1]+1, 4]==0)
Do[r=0; Do[If[Mod[n-x(2x-1)-y(2y-1)-2z(2z-1), 4]==0&&H[(n-x(2x-1)-y(2y-1)-2z(2z-1))/4], r=r+1], {x, 0, (Sqrt[4n+1]+1)/4}, {y, x, (Sqrt[8(n-x(2x-1))+1]+1)/4},
{z, 0, (Sqrt[4(n-x(2x-1)-y(2y-1))+1]+1)/4}]; Print[n, " ", r]; Continue, {n, 0, 100}]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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