Let $k$ be an algebraically closed field of characteristic p. Let $Z\subset k[x_1,\cdots,x_n]$ be a graded $k$-subalgebra of a polynomial ring, such that for any $f\in Z,$ any divisor of $f$ (in $k[x_1,\cdots,x_n]$) is again an element of $Z.$ Then is $Z[x_1^p,\cdots,x_n^p]$ a normal ring? For the easiest case of $Z=k[f]-$where $f$ is an irreducible homogeneous polynomial, the answer is yes, since $Z[x_1^p,\cdots,x_n^p]$ in this case is a complete intersection and regular in codimension 2.
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$\begingroup$ Welcome new contributor. I do not understand your assertion. Consider the case that $n$ equals $2$ and $Z$ equals the subalgebra of $k[x_1,x_2]$ generated by $f=x_1^p + x_2^{p+1}$. Then $Z[x_1^p,x_2^p]$ equals the $k$-subalgebra $k[x_1^p,x_2^p,x_2^{p+1}]$ of $k[x_1,x_2]$. This $k$-subalgebra is not normal, it is not a complete intersection scheme, and it is not regular in codimension $1$, much less in codimension $2$. $\endgroup$– Jason StarrCommented Feb 11, 2019 at 2:36
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$\begingroup$ Thank you, I forgot to add that Z is graded subalgebra, I hope this at least saves the question for Z=k[f]. $\endgroup$– John Zek.Commented Feb 11, 2019 at 2:52
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1$\begingroup$ Welcome new contributor. I still do not understand your assertion. If you insist that $f$ be homogeneous, then please consider the case that $n$ equals $3$ and that $Z$ is the graded $k$-subalgebra of $k[x_1,x_2,x_3]$ generated by $f=x_1^p - x_2^{p-1}x_3$. Then $Z[x_1^p,x_2^p,x_3^p]$ equals $k[x_1^p,x_2^p,x_3^p,x_2^{p-1}x_3]$. This $k$-algebra is not normal, and it is not regular in codimension $1$. $\endgroup$– Jason StarrCommented Feb 11, 2019 at 12:38
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$\begingroup$ Thank you again, clearly I've been making a silly mistake. $\endgroup$– John Zek.Commented Feb 11, 2019 at 16:19
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