OFFSET
1,2
COMMENTS
Conjecture: (i) a(n) > 0 for all n > 0, and a(n) = 1 only for n = 1, 9, 13, 89, 449, 524, 1049, 2164, 14969, 51334.
(ii) For any positive integer n, there are integers x and y > 0 such that n - x^4 - T(y) has the form T(z) or 4*T(z), where T(k) refers to the triangular number k*(k+1)/2.
(iii) For every n = 1,2,3,... there are integers x and y > 0 such that n - x^4 - T(y) has the form T(z) or 2*z^2.
(iv) For {c,d} = {1,2} and n > 0, there are integers x and y > 0 such that n - 2*x^4 - T(y) has the form c*T(z) or d*z^2.
(v) For each n = 1,2,3,... there are integers x and y > 0 such that n - 4*x^4 - T(y) has the form 2*T(z) or z^2.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 1..10000
Zhi-Wei Sun, Mixed sums of squares and triangular numbers, Acta Arith. 127(2007), 103-113.
EXAMPLE
a(1) = 1 since 1 = 0^4 + 1*2/2 + 0^2.
a(9) = 1 since 9 = 0^4 + 1*2/2 + 8*1^2.
a(13) = 1 since 13 = 1^4 + 2*3/2 + 3^2.
a(89) = 1 since 89 = 2^4 + 1*2/2 + 8*3^2.
a(449) = 1 since 449 = 0^4 + 22*23/2 + 14^2.
a(524) = 1 since 524 = 3^4 + 29*30/2 + 8*1^2.
a(1049) = 1 since 1049 = 5^4 + 16*17/2 + 8*6^2.
a(2164) = 1 since 2164 = 1^4 + 34*35/2 + 8*14^2.
a(14969) = 1 since 14969 = 8^4 + 145*146/2 + 8*6^2.
a(51334) = 1 since 51334 = 5^4 + 313*314/2 + 8*14^2.
MATHEMATICA
SQ[n_]:=IntegerQ[Sqrt[n]]||IntegerQ[Sqrt[n/8]]
Do[r=0; Do[If[SQ[n-x^4-y(y+1)/2], r=r+1], {x, 0, n^(1/4)}, {y, 1, (Sqrt[8(n-x^4)+1]-1)/2}]; Print[n, " ", r]; Continue, {n, 1, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Oct 05 2015
STATUS
approved