The Left-Nilpotent Residual and Its Supplements in Finite Skew Braces

Abstract

Let $X$ be a finite skew brace, and let $L_\infty(X)$ be the final term of its left series. We introduce the left-nilpotent residual $\RL(X)$ and prove that $ \RL(X)=\Id_X\bigl(L_\infty(X)\bigr). $ Thus $X/\RL(X)$ is the largest left-nilpotent quotient of $X$. For skew braces of nilpotent type, we relate $\RL(X)$ to $\gamma_\infty(X,\cdot)$ and obtain conditions ensuring that $L_\infty(X)$ is an ideal, including the case where $|X|$ is cube-free. Our main results concern supplements to $\RL(X)$. We prove existence results using coprime action and Sylow and Hall theory, and give an example of order $18$ whose residual has no proper supplement. Finally, iterating $\RL$ yields the largest perfect subskew brace of $X$, and $X$ is KSV-solvable if and only if this subskew brace is zero.