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An old problem (recently reheated" by Casas-AlvarezCasas-Alvero, etc.) consists of trying to determine if degree $n$ polynomials $P(t)$ that are $n$-th powers are characterized by the following condition:

$$\deg( gcd(P(t),P^{(r)}(t))) >0$$ for all positive integers $r$ and $r \neq n,$ where $P^{(r)}(t)$ is the $r$-th formal derivative of $P(t)$ relative to $t$.

Seems that the case when the field of coefficients is the prime field $GF(p)$ has not been considered.

By chance I got the following polynomial $P_{37}(t) \in GF(37)[t]$ that proves the (conjectural) result false:

$$P_{37}(t) = 17t^{12}+21t^4+24t^3+12t^2 \in GF(37)[t]$$

We have indeed:

$$\deg(\gcd(P_{37}(t),P_{37}^{(r)}(t)) \geq 1$$ in $GF(37)[t]$ for all $r=1,2, \ldots,11$ and trivially for $r>13.$ But,

$$P_{37}(t)$$ is not a $12$-th power in $GF(37)[t].$

More precisely: $$\gcd(P(t),P^{'}(t))=t, \quad \gcd(P(t),P^{''}(t)= t+36,$$ $$\gcd(P(t),P^{(3)}(t))=t+36, \quad \gcd(P,P^{(4)}(t)=t+36,$$ and all other $\gcd(P(t),P^{(r)}(t))=t$ for $r=5, \ldots,11$ and for $r>12.$

Question: There are other such examples ?. Can be characterized ?

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# Characterizing degree $n$ polynomials that are $n$-th powers in $GF(p)[t]$

An old problem (recently reheated" by Casas-Alvarez, etc.) consists of trying to determine if degree $n$ polynomials $P(t)$ that are $n$-th powers are characterized by the following condition:

$$\deg( gcd(P(t),P^{(r)}(t))) >0$$ for all positive integers $r$ and $r \neq n,$ where $P^{(r)}(t)$ is the $r$-th formal derivative of $P(t)$ relative to $t$.

Seems that the case when the field of coefficients is the prime field $GF(p)$ has not been considered.

By chance I got the following polynomial $P_{37}(t) \in GF(37)[t]$ that proves the (conjectural) result false:

$$P_{37}(t) = 17t^{12}+21t^4+24t^3+12t^2 \in GF(37)[t]$$

We have indeed:

$$\deg(\gcd(P_{37}(t),P_{37}^{(r)}(t)) \geq 1$$ in $GF(37)[t]$ for all $r=1,2, \ldots,11$ and trivially for $r>13.$ But,

$$P_{37}(t)$$ is not a $12$-th power in $GF(37)[t].$

Question: There are other such examples ?. Can be characterized ?