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I don't think that $p$ being prime makes any difference. None of what I am about to write is news but it The later thoughts below suggest a Lagrange interpolation method which is worth sayingperhaps the same as the resultant method mentioned by Abdelmalek Abdesselam.

Let $f(x)=\sum_{j=0}^nc_jx^j.$ One might choose to have require $c_n=1$ or $c_0=1$ but it is perhaps nicer not to. Then setting $u=x^n,$ $\prod_{i=0}^{n-1} f(\omega^ix)=\sum_{j=0}^nC_j(x^n)^j$f(\omega^ix)=F(u)=\sum_{j=0}^nC_j u^j$. The • the roots of$F$are the$n$th powers of the roots of$f$. • Here$c_n$is a constant and the other$c_i$are symmetric polynomials of the$n$roots$\alpha_i$of$f$( for $i<n$,$c_n$is a constant). f$. The $\alpha_i$ can be thought of as formal variables. Then $c_0,\cdots,c_{n-1}$ are also a basis for the ring of all symmetric polynomials in those variables ($\frac{1}{c_n}$ times a usual basis) . There are other bases for this ring such as $\sigma_k=\sum_{i=1}^n\alpha_i^k$ and the sum of all the terms $\binom{n}{k}$ products of \alpha_1^{m_1}\cdots\alpha_n^{m_n}$with$k$roots. m_1+\cdots+m_n=k$. Transforming between these bases (mre more generally, expressing a given symmetric polynomial in terms of them) is a major topic of invariant theory.

In this case, we want to express the $C_i$, which are certain symmetric polynomials in the $\alpha_i^n$ (scaled by a power \alpha_i^n,$ in terms of the values$c_n$). c_i$. This must be a well known case. At any rate here are some results:

$$C_2=(6c_0^2c_4^2-8c_0c_1c_3c_4+2c_1^2c_3^2)+(4c_0c_3^2+4c_1^2c_4)c_2+(-4c_0c_4-4c_1c_3)c_2^2+c_2^4$$$C_2=(6c_0^2c_4^2-8c_0c_1c_3c_4+2c_1^2c_3^2)+(4c_0c_3^2+4c_1^2c_4)c_2-(4c_0c_4+4c_1c_3)c_2^2+c_2^4{\small C_2=(10c_0^3c_5^2-15c_0^2c_1c_4c_5+5c_0c_1^2c_4^2+5c_0^2c_3^2c_4+10c_0c_1^2c_3c_5-5c_0c_1c_3^3-5c_1^3c_3c_4)+(5c_0^2c_4^2-15c_0^2c_3c_5+5c_1^2c_3^2-5c_0c_1c_3c_4-5c_1^3c_5)c_2}$$C_2=(10c_0^3c_5^2-15c_0^2c_1c_4c_5+5c_0c_1^2c_4^2+5c_0^2c_3^2c_4+10c_0c_1^2c_3c_5-5c_0c_1c_3^3-5c_1^3c_3c_4)}$$ $${ \small+(5c_0^2c_4^2-15c_0^2c_3c_5+5c_1^2c_3^2-5c_0c_1c_3c_4-5c_1^3c_5)c_2}$$ $${\small +(5c_1^2c_4+10c_0c_1c_5+5c_0c_3^2)c_2^2-(5c_0c_4+5c_1c_3)c_2^3+c_2^5}$$ later thoughts In general one could consider the problem of producing from a polynomial$f(x)=\sum_{j=0}^nc_jx^j$a polynomial$F(u)=\sum_{j=0}^nC_j u^j$whose roots are the$m$th powers$\alpha_i^m$of the$n$(unknown) roots of$f$. The solution is the polynomial$\prod_{i=0}^{m-1} f(\tau^ix)$where now$\tau$is a primitive$m$th root of unity. This process might spread out the roots. In the case$m=2$one has (with$ q$repeated application)$\alpha_i^{2^q}$and the Dandelinâ€“Graeffe method for finding the roots of a univariate polynomial. Splitting$f$into its even and odd parts speeds up the computation. The method was also discovered by Nikolai Ivanovich Lobachevsky and the linked article suggests that his book Algebra ili Ichislenie Konechnykh Velichin discussed the general product$\prod_{i=0}^{m-1} f(\omega^ix)$. Perhaps the appropriate manipulations of symmetric polynomials are discussed there. Since a polynomial of degree$n$is determined by its values at$n$points (plus its leading coeffcient) one has the following method (which I doubt is new):Let$\zeta$be a primitive$mn$th root of unity and$\omega$a primitive$m$th root of unity. Then the desired polynomial$F$satisfies$F(\zeta^j)=f(\omega^j)$for$0 \le j \le n-1$. Now Lagrange Interpolation can be used. In this case it might be particularly simple. 4 deleted 10 characters in body I don't think that$p$being prime makes any difference. None of what I am about to write is news but it is worth saying. Let$f(x)=\sum_{j=0}^nc_jx^j. $One might choose to have$c_n=1$or$c_0=1$but it is perhaps nicer not to. Then$\prod_{i=0}^{n-1} f(\omega^ix)=\sum_{j=0}^nC_j(x^n)^j$. One can say that •$C_j$is a polynomial of degree$n$in the coefficients$c_0,\cdots,c_n$where each term has total degree$n$•$C_j$has a term$(-1)^{nj}(c_j)^n$\pm(c_j)^n$ and no term $c_j^{n-1}$.
• $C_{n-j}$ is $C_j$ with $c_k$ replaced by $c_{n-k}$

The $c_i$ are symmetric polynomials of the $n$ roots $\alpha_i$ of $f$ ( for $i<n$, $c_n$ is a constant). The $\alpha_i$ can be thought of as formal variables. Then $c_0,\cdots,c_{n-1}$ are also a basis for the ring of all symmetric polynomials ($\frac{1}{c_n}$ times the a usual basis) . There are other bases for this ring such as $\sigma_k=\sum_{i=1}^n\alpha_i^k$ and the sum of all $\binom{n}{k}$ products of $k$ roots. Transforming between these bases (mre generally, expressing a given symmetric polynomial in terms of them) is a major topic of invariant theory.

In this case we want to express certain symmetric polynomials in the $\alpha_i^n$ (scaled by a power of $c_n$). This must be a well known case. At any rate here are some results:

For $n=4,$

$$C_0=c_0^4$$

$$C_1=(4c_0^3c_4-2c_0^2c_2^2)-(4c_0^2c_3)c_1+(4c_0c_2)c_1^2-c_1^4$$

$$C_2=(6c_0^2c_4^2-8c_0c_1c_3c_4+2c_1^2c_3^2)+(4c_0c_3^2+4c_1^2c_4)c_2+(-4c_0c_4-4c_1c_3)c_2^2+c_2^4$$

$$C_3=(4c_4^3c_0-2c_4^2c_2^2)-(4c_4^2c_1)c_3+4(c_4c_2)c_3^2-c_3^4$$

$$C_4=c_4^4$$

while for $n=5$ we have the following (with the others obtainable by symmetry)

$$C_0=c_0^5$$

$$C_1=5c_0^4c_5-(5c_0^3c_4-5c_0^2c_2^2)c_1+(5c_0^2c_3)c_1^2-5c_0^3c_2c_3-(5c_0c_2)c_1^3+c_1^5$$

$${\small C_2=(10c_0^3c_5^2-15c_0^2c_1c_4c_5+5c_0c_1^2c_4^2+5c_0^2c_3^2c_4+10c_0c_1^2c_3c_5-5c_0c_1c_3^3-5c_1^3c_3c_4)+(5c_0^2c_4^2-15c_0^2c_3c_5+5c_1^2c_3^2-5c_0c_1c_3c_4-5c_1^3c_5)c_2}$$ $${\small +(5c_1^2c_4+10c_0c_1c_5+5c_0c_3^2)c_2^2+(-5c_0c_4-5c_1c_3)c_2^3+c_2^5}$$(5c_1^2c_4+10c_0c_1c_5+5c_0c_3^2)c_2^2-(5c_0c_4+5c_1c_3)c_2^3+c_2^5}$$3 deleted 3 characters in body; added 1 characters in body I don't think that p being prime makes any difference.None difference. None of what I am about to write is news but it is worth saying. Let f(x)=\sum_{j=0}^nc_jx^j.  One might choose to have c_n=0 c_n=1 or c_0=0 c_0=1 but it is perhaps nicer not to. Then \prod_{i=0}^{n-1} f(\omega^ix)=\sum_{j=0}^nC_j(x^n)^j. One can say that • C_j is a polynomial of degree n in the coefficients c_0,\cdots,c_n where each term has total degree n • C_j has a term (-1)^{nj}(c_j)^n and no term c_j^{n-1}. • C_{n-j} is C_j with c_k replaced by c_{n-k} The c_i are symmetric polynomials of the n roots \alpha_i of f ( for i<n, c_n is a constant). The \alpha_i can be thought of as formal variables. Then c_0,\cdots,c_{n-1} are also a basis for the ring of all symmetric polynomials (\frac{1}{c_n} times the a usual basis) . There are other bases for this ring such as \sigma_k=\sum_{i=1}^n\alpha_i^k and the sum of all \binomial{n}{k} \binom{n}{k} products of k roots. Transforming between these bases (mre generally, expressing a given symmetric polynomial in terms of them) is a major topic of invariant theory. In this case we want to express certain symmetric polynomials in the \alpha_i^n (scaled by a power of c_n). This must be a well known case. At any rate here are some results: For n=4,$$C_0=c_0^4C_1=(4c_0^3c_4-2c_0^2c_2^2)-(4c_0^2c_3)c_1+(4c_0c_2)c_1^2-c_1^4C_2=(6c_0^2c_4^2-8c_0c_1c_3c_4+2c_1^2c_3^2)+(4c_0c_3^2+4c_1^2c_4)c_2+(-4c_0c_4-4c_1c_3)c_2^2+c_2^4C_3=(4c_4^3c_0-2c_4^2c_2^2)-(4c_4^2c_1)c_3+4(c_4c_2)c_3^2-c_3^4C_4=c_4^4$$while for n=5 we have the following (with the others obtainable by symmetry)$$C_0=c_0^5C_1=5c_0^4c_5-(5c_0^3c_4-5c_0^2c_2^2)c_1+(5c_0^2c_3)c_1^2-5c_0^3c_2c_3-(5c_0c_2)c_1^3+c_1^5{\small C_2=(10c_0^3c_5^2-15c_0^2c_1c_4c_5+5c_0c_1^2c_4^2+5c_0^2c_3^2c_4+10c_0c_1^2c_3c_5-5c_0c_1c_3^3-5c_1^3c_3c_4)+(5c_0^2c_4^2-15c_0^2c_3c_5+5c_1^2c_3^2-5c_0c_1c_3c_4-5c_1^3c_5)c_2}{\small +(5c_1^2c_4+10c_0c_1c_5+5c_0c_3^2)c_2^2+(-5c_0c_4-5c_1c_3)c_2^3+c_2^5}

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