= Sums of two values of x cubed plus three x over the 3-adic integers
{title2=$n\equiv0,\pm1,\pm4\pmod9$}
For $n\in\mathbb Z_3$, the equation $x^3+3x+y^3+3y=n$ has a solution in the <p-adic integers> exactly for the displayed residue classes. The polynomial $F(t)=t^3+3t$ has values $0,4,5$ modulo nine, whose pairwise sums are $0,1,4,5,8$. For sufficiency choose initial pairs $(0,0),(-1,-1),(1,0),(-1,0),(1,1)$ for those five classes respectively, fix the second coordinate, and lift the first. Its derivative is $3(x^2+1)$, always of 3-adic norm $1/3$, while its second Taylor coefficient is $3x$, of norm below one. The residual is divisible by nine, so either the strict or the <Boundary case of Hensel's lemma with a small quadratic coefficient> applies.
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