Innerness criterion for twisted derivations when the conductor is 2rp2^r p

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Let K=Q(ζ)\mathbb{K}=\mathbb{Q}(\zeta) be the nnth cyclotomic number field with n=2rpn=2^r p, where r∈Nr\in\mathbb{N} and pp is an odd rational prime, and let OK=Z[ζ]O_{\mathbb{K}}=\mathbb{Z}[\zeta]. Let u,v∈U(n)u,v\in U(n) with u≠vu\ne v, let σ(ζ)=ζu\sigma(\zeta)=\zeta^u and τ(ζ)=ζv\tau(\zeta)=\zeta^v, and let AA be the coefficient matrix from the preceding determinant conjecture. For a (σ,τ)(\sigma,\tau)-derivation D:OK→OKD:O_{\mathbb{K}}\to O_{\mathbb{K}}, write

D(ζ)=∑i=02r−1(p−1)−1ciζi∈OK,CT=(c0,…,c2r−1(p−1)−1).D(\zeta)=\sum_{i=0}^{2^{r-1}(p-1)-1}c_i\zeta^i\in O_{\mathbb{K}},\qquad C^T=(c_0,\ldots,c_{2^{r-1}(p-1)-1}).

Innerness conjecture. Under the hypotheses of the determinant conjecture, if 1≤e1≤r−11\le e_1\le r-1 and e2≥1e_2\ge1, then DD is inner if and only if

122e1(p−1)Adj⁡(A)C∈Z2r(p−1).\frac{1}{2^{2^{e_1}(p-1)}}\operatorname{Adj}(A)C\in\mathbb{Z}^{2^r(p-1)}.

In particular, divisibility of every cic_i by 22e1(p−1)2^{2^{e_1}(p-1)}, and even divisibility of every cic_i by 22, implies innerness. If e1≥re_1\ge r and e2=0e_2=0, replace the denominator by p2r−1p^{2^{r-1}}; in all other cases DD is always inner. This is the proposed necessary-and-sufficient characterization of inner twisted derivations for this conductor form; together with the determinant claim, it is intended to distinguish the cases where outer derivations can occur.

References

Primary source

Praveen Manju and Rajendra Kumar Sharma, “Twisted Derivations in Algebraic Number Fields”, arXiv:2412.03507 (2026).

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