Sine laws on semigroups with an involutive anti-automorphism: A Levi--Civita approach via left translations

Abstract

Stetkær's matrix (Levi--Civita) method is a powerful tool for functional equations on semigroups involving a homomorphism $\sigma$, as it yields a finite-dimensional invariant space under right translations and a corresponding matrix formalism. When $\sigma$ is an involutive anti-automorphism, however, the parametrized family of right translations reverses multiplication order in the parameter. In this paper, we resolve this operator-level mismatch by establishing the conjugation identity: letting $J$ denote composition with $\sigma$, we prove \[ J\,R(σ(y))\,J=L(y)\qquad(\forall\,y\in S), \] which converts the problematic right translates into left translations. Using this left-translation approach, we obtain an anti-automorphic Levi--Civita closure principle and apply it to the generalized sine law. The classical dichotomy $\beta \in \{\pm 1\}$ and the parity relation $f\circ \sigma =\beta f$ are obtained without the bridge hypothesis. Under a natural bridge hypothesis, which is automatically satisfied when there exists a central element $c$ with $f(c)\ne 0$, we obtain the corresponding standard $xy$-addition law and the exact $\sigma$-transformation rule for $g$.

12 pages. Some minor errors have been corrected in this final version. To appear in Acta Mathematica Hungarica (2026)