OFFSET
1,2
COMMENTS
Conjecture: (i) a(n) > 0 for all n > 0.
(ii) Any positive integer n can be written as x^3 + 2*y^3 + 4*z^3 + p(k) with x,y,z nonnegative integers and k a positive integer.
(iii) For each c = 3, 4, any positive integer n can be written as x^3 + 2*y^3 + c*z^3 + A000009(k) with x,y,z nonnegative and k a positive integer.
We have verified the conjecture for n up to 1.3*10^6.
On the author's request, Prof. Qing-Hu Hou at Tianjin Univ. has verified all the three parts of the conjecture for n up to 10^9. - Zhi-Wei Sun, Feb 06 2017
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1) = 1 since 1 = 0^3 + 2*0^3 + 3*0^3 + p(1).
a(2) = 2 since 2 = 1^3 + 2*0^3 + 3*0^3 + p(1) = 0^3 + 2*0^3 + 3*0^3 + p(2).
a(75) = 3 since 75 = 4^3 + 2*0^3 + 3*0^3 + p(6) = 3^3 + 2*1^3 + 3*2^3 + p(8) = 0^3 + 2*2^3 + 3*1^3 + p(11).
MATHEMATICA
CQ[n_]:=CQ[n]=IntegerQ[n^(1/3)]
p[n_]:=p[n]=PartitionsP[n]
Do[r=0; Do[If[p[k]>n, Goto[bb]]; Do[If[CQ[n-p[k]-3x^3-2y^3], r=r+1], {x, 0, ((n-p[k])/3)^(1/3)}, {y, 0, ((n-p[k]-3x^3)/2)^(1/3)}]; Continue, {k, 1, n}]; Label[bb]; Print[n, " ", r]; Continue, {n, 1, 80}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Jan 31 2017
STATUS
approved