OFFSET
0,3
COMMENTS
Conjecture: a(n) > 0 for all n. Also, a(n) = 1 only for n = 0, 1, 4, 6, 10, 11, 23.
This has been verified for all n = 0..10^7. We have proved that every nonnegative integer can be written as x*(x+1) + y*(3*y+1)/2 + z*(3*z-1)/2 with x,y,z integers.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 0..10000
Zhi-Wei Sun, On universal sums of polygonal numbers, arXiv:0905.0635.
EXAMPLE
a(10) = 1 since 10 = 1*2 + 2*(3*2+1)/2 + 1*(3*1-1)/2.
a(11) = 1 since 11 = 2*3 + 0*(3*0+1)/2 + 2*(3*2-1)/2.
a(23) = 1 since 23 = 4*5 + 1*(3*1+1)/2 + 1*(3*1-1)/2.
a(34) = 2 since 34 = 3*4 + 0*(3*0+1)/2 + 4*(3*4-1)/2 = 4*5 + 1*(3*1+1)/2 + 3*(3*3-1)/2.
MATHEMATICA
sQ[n_]:=IntegerQ[Sqrt[4n+1]]
Do[r=0; Do[If[sQ[n-y(3y+1)/2-z(3z-1)/2], r=r+1], {y, 0, (Sqrt[24n+1]-1)/6}, {z, 0, (Sqrt[24(n-y(3y+1)/2)+1]+1)/6}];
Print[n, " ", r]; Continue, {n, 0, 70}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Feb 01 2015
STATUS
approved