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At 1st we consider some weak statement of Chevalley–Warning theorem for any finite field: If $f$ is a homogeneous polynomial of degree $d$ with $n$ independent variables over a finite field $F$. Then if $ n > d $ then there is a non trivial solution of this homogeneous polynomial in $ F^{n} / \{0,0,...,0\} $.

Now if we consider $f$ is a homogeneous polynomial of degree $d$ with $n$ independent variables over the rational field $\mathbb{Q}$ and if $ n > d $ does there exist always a nontrivial solution in $ \mathbb{Q}^{n} / \{0,0,...,0\} $? The answer is no when $r$ is even as for an example $ x_{1}^{d} + x_{2}^{d}+.....+ x_{n}^{d} = 0 $ have only one solution $\{0,0,...,0\} $ in $ \mathbb{Q}^{n}$.

My question is when $ d $ is odd does there exist always a nontrivial solution in $ \mathbb{Q}^{n} / \{0,0,...,0\} $? Describe when $ d = 3 $ and $ n= 5 $ especially.

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    $\begingroup$ In the statement of CW theorem, “nontrivial” just means “not $(0,\ldots,0)$”. You do not say what you want “nontrivial” to mean. If it just means “not identically $0$” then of course there are solutions not identically $0$ for odd $r$ when $n\geq 2$, namely $(1,-1,0,\ldots,0)$. $\endgroup$
    – KConrad
    Aug 10, 2021 at 7:21
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    $\begingroup$ That is not true. In fact, there are often no “local” points after base change to the bigger field $\mathbb{Q}_p$. Emil Artin conjectured existence of local points under the stronger hypothesis that $n >d^2$. For fixed $n$ and $d$, and for $p$ sufficiently positive this was proved by Ax and Kochen. For small $p$ the first counterexamples were found by Terjanian. $\endgroup$ Aug 10, 2021 at 8:51
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    $\begingroup$ There are cubic forms in 9 variables which don't represent zero nontrivially, but any form in 14 variables, nontrivial zeros exist. Analogous results exist for other odd $r$, see here $\endgroup$
    – Wojowu
    Aug 10, 2021 at 8:52
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    $\begingroup$ Terjanian’s counterexample is on p. 2 of the following expository article of Heath-Brown: arxiv.org/pdf/1002.3754.pdf $\endgroup$ Aug 10, 2021 at 10:03
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    $\begingroup$ If you want examples with arbitrary degree $d$ in $n=d^2$ variables with no local points at prime $p$, consider the reduced norm of an Azumaya algebra of period equals index $d$ whose base change is a generator of the $d$-torsion subgroup of the Brauer group of the local field (as in class field theory). $\endgroup$ Aug 10, 2021 at 10:07

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The condition that $d$ is odd just implies that there is no real obstruction to the existence of rational points, but there could still be $p$-adic obstructions for some prime $p$.

As an example, let $p$ be a prime and $a$ an integer which is coprime to $p$ for which $a \bmod p$ is not a cube (this necessarily implies that $p \equiv 1 \bmod 3$).

Consider the cubic surface: $$X: \quad x_1^3 - a x_2^3 + p(x_3^3 - a x_4^3) = 0.$$

It is a fun exercise to show that this has no non-trivial rational solution; the proof actually shows that there is no non-trivial $p$-adic solution. Note that there is obviously a non-trivial solution mod $p$, e.g. $(0,0,1,1) \bmod p$, as predicted by Chevalley-Warning.

A more interesting question is whether having a real point and a $p$-adic point is sufficient to guarantee a rational point; again this is not true in general, with smooth cubic surfaces giving counter-examples (this is called the Hasse principle in the literature). A famous example, due to Cassels and Guy, is: $$5x_1^3 + 12x_2^3 + 9x_3^3 + 10x_4^3 = 0.$$ Proving that there is no non-trivial rational point in this case is not so elementary (it requires cubic reciprocity). From a modern perspective there is a Brauer--Manin obstruction to the Hasse principle in this case.

I suspect from the question you may have a specific example in mind you are studying. If you allow many variables (roughly $d2^d$) then the circle method can actually show that the Hasse principle holds, which may be sufficient for your application.

Addendum: This is a summary of the alternative construction given in the comments, written up for completeness.

Let $d \in \mathbb{N}$. Let $D$ be a division algebra over $\mathbb{Q}$ of dimension $d^2$. This can be shown to exist using the fundamental exact sequence for the Brauer group of $\mathbb{Q}$ from class field theory, combined with the fact that the period = index for central simple algebras over number fields.

Consider the reduced norm on $D$. After choosing a $\mathbb{Q}$-basis for $D$, this defines a homogeneous polynomial $N$ of degree $d$ in $d^2$ variables. We claim that the equation $N(\mathbf{x}) = 0$ has no non-trivial solution.

To see this, we use the fact that an element of a central simple algebra has reduced norm $0$ if and only if it is not invertible. But by construction $D$ is a division algebra, hence every non-zero element is invertible, hence only $\mathbf{0}$ has $0$ norm, as claimed.

Note: Division algebras satisfy the Hasse principle, hence this failure of existence of rational solutions is explained by a failure of existence of $p$-adic solutions for some $p$.

It is possible, though painful, to make these norm form equations explicit. The first issue is writing down explicit division algebras, which Will Sawin did in the comments.

As an example, consider the quaternion algebra $(a,b)$. The reduced norm here is just the Pfister form: $$x_1^2 - ax_2^2 - bx_3^2 + abx_4^2.$$ Taking $b = p $ to be an odd prime and $a<0$ such that $a \bmod p$ is a quadratic non-residue, one obtains an example which does not non-trivially represent zero $p$-adically, but has real solutions. (The above cubic surface example is a variant of this equation).

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    $\begingroup$ It is worth commenting (as @DanielLoughran knows), that Colliot-Th'el`ene has asked the question, for geometrically rational, unirational, rationally connected, etc. varieties over a global field, whether the Brauer-Manin obstruction might be the only obstruction to the Hasse principle (one could even ask whether rational points are dense in the Brauer-Manin subset of the adelic points). $\endgroup$ Aug 10, 2021 at 11:48
  • $\begingroup$ Dear @Daniel this is very useful example for 4 variables can you give any cubic 5 variables which has only trivial solution? Does there any method to check an arbitrary cubic homogeneous poly in 4 or 5 variables which have only trivial solution $\endgroup$
    – Sky
    Aug 14, 2021 at 19:33
  • $\begingroup$ @SugataMandal: It's quite easy to come up with variants of my example, for example $x_1^3 - ax_2^3 + p(x_3^3 - ax_4^3) + p^2x_5^3 = 0$. $\endgroup$ Aug 15, 2021 at 14:46
  • $\begingroup$ As for your second question, the answer is no: The first thing one checks is for solutions in every $p$-adic field. For 5 variables this is conjectured to be sufficient (at least for smooth cubics), but this is not known in general. For 4 variables the Hasse principle can fail as I explain in my answer, and it is not known in general whether the Brauer-Manin obstruction is the only one to the Hasse principle. $\endgroup$ Aug 15, 2021 at 14:48
  • $\begingroup$ But there are partials results and methods which work in special cases. So if you have some specific equation in mind, it might be possible to handle it. $\endgroup$ Aug 15, 2021 at 14:49

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