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

From papers

Let K=Q(ζ)\mathbb{K}=\mathbb{Q}(\zeta) be the nnth cyclotomic number field with n=2rpn=2^r p, where rNr\in\mathbb{N} and pp is an odd rational prime, and let OK=Z[ζ]O_{\mathbb{K}}=\mathbb{Z}[\zeta]. Let u,vU(n)u,v\in U(n) with uvu\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:OKOKD:O_{\mathbb{K}}\to O_{\mathbb{K}}, write

D(ζ)=i=02r1(p1)1ciζiOK,CT=(c0,,c2r1(p1)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 1e1r11\le e_1\le r-1 and e21e_2\ge1, then DD is inner if and only if

122e1(p1)Adj(A)CZ2r(p1).\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(p1)2^{2^{e_1}(p-1)}, and even divisibility of every cic_i by 22, implies innerness. If e1re_1\ge r and e2=0e_2=0, replace the denominator by p2r1p^{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.

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Primary source

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

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