Equivariant Nijenhuis Lie Algebras: Extensions to Classical Lie-Theoretic Structures

Abstract: We develop a structural theory of equivariant Nijenhuis Lie algebras (ENL algebras), namely, Lie algebras equipped with Nijenhuis operators satisfying an equivariance condition with respect to the adjoint representation. This rigidity allows classical Lie bialgebra constructions to extend systematically to the operator-equipped setting. Within this framework, we define ENL bialgebras and establish the associated notions of matched pairs, Manin triples, and Drinfel'd doubles. We show that coboundary ENL bialgebras are characterized by EN $r$-matrices satisfying an equivariant classical Yang-Baxter equation. We further introduce EN-relative Rota-Baxter operators and prove that they provide an operator-theoretic realization of such $r$-matrices, leading to descendent ENL algebras and to solutions of the classical Yang--Baxter equation on semidirect ENL algebras. In the quadratic case, this construction reduces to Rota-Baxter operators of weight zero. Finally, we extend the EN framework to pre-Lie algebras and show that pre-ENL algebras naturally induce associated ENL structures.