On integer solutions to $a^3+b^3+c^3+d^3=3$ I was challenged to prove that there are infinitely many solutions to the equation$$a^3+b^3+c^3+d^3=3\ \ \text{ with }(a,b,c,d)\in\mathbb Z^4$$
That was easy: elementary algebra is enough to prove that $\forall z\in \mathbb Z$ then $$(a,b,c,d)=(1+6z^3,\,1-6z^3,\,-6z^2,\,1)$$ is a solution. Call these solutions and their permutations trivial.
Define a nonsum as a value of $k\in\mathbb Z$ for which there is no solution with $a+b=k$.
What about proving (or disproving/perfecting) the following conjectures?

*

*If polynomials $A$, $B$, $C$, $D$ with integer coefficients are such that $\forall z\in \mathbb Z,\ (A(z),B(z),C(z),D(z))$ is a solution, and one such solution is non-trivial, then all the polynomials are constant.

*There are nevertheless infinitely many non-trivial solutions.

*Any $k\equiv0\pmod3$ is a nonsum.

*$k=7$ is a nonsum.

*There are infinitely many nonsums with $k\not\equiv0\pmod3$

*If $k$ is a nonsum, then $-k$ is a nonsum.

*If $k$ is not a nonsum (that is, if there exists a solution with $a+b=k$), then there are infinitely many solutions with $a+b=k$.


Conjecture 3 is true.
Note first that for any integer $n$, $n^3 \equiv -1,0$, or $1\pmod 9$. Hence any solution of $a^3+b^3+c^3+d^3=3$ must contain three cubes congruent to $1\pmod 9$ and one cube congruent to $0\pmod9$.
Now suppose $a+b = k\equiv0\pmod3$. We have:
$$a^3+b^3=(a+b)^3-3ab(a+b)$$
where:
$$(a+b)^3\equiv0\pmod{3^3}\quad \text{and}\ \textit{a fortiori}\quad \equiv0\pmod9$$
$$3ab(a+b)\equiv0\pmod9$$
Hence $a^3+b^3\equiv0\pmod9$ which is inconsistent with the condition stated above, since it implies either both $a^3$ and $b^3$ congruent to $0\pmod9$ or one of them congruent to $-1$ and the other to $1\pmod9$.
Similar but not identical reasoning shows that conjecture 4 is true, and more generally any $k\equiv0\pmod7$ is a nonsum.
For any integer $n$, $n^3 \equiv -1,0$, or $1\pmod 7$. Hence any solution of $a^3+b^3+c^3+d^3=3$ must contain either (Case 1) three cubes congruent to $1\pmod 7$ and one cube congruent to $0\pmod7$ or (Case 2) four cubes congruent to $-1\pmod7$
Now suppose $a+b = k\equiv0\pmod7$. We have:
$$a^3+b^3=(a+b)(a^2-ab+b^2)\equiv0\pmod7$$
But this is inconsistent with both Case 1 and Case 2 above, since it implies either both $a^3$ and $b^3$ congruent to $0\pmod7$ or one of them congruent to $-1$ and the other to $1\pmod7$.
