Invariants and representations of the $Γ$-graded general linear Lie $ω$-algebras

There is considerable current interest in applications of generalised Lie algebras graded by an abelian group $Γ$ with a commutative factor $ω$. This calls for a systematic development of the theory of such algebraic structures. We treat the representation theory and invariant theory of the $Γ$-graded general linear Lie $ω$-algebra $\mathfrak{gl}(V(Γ, ω))$, where $V(Γ, ω)$ is any finite dimensional $Γ$-graded vector space. Generalised Howe dualities over symmetric $(Γ, ω)$-algebras are established, from which we derive the first and second fundamental theorems of invariant theory, and a generalised Schur-Weyl duality. The unitarisable $\mathfrak{gl}(V(Γ, ω))$-modules for two ``compact'' $\ast$-structures are classified, and it is shown that the tensor powers of $V(Γ, ω)$ and their duals are unitarisable for the two compact $\ast$-structures respectively. A Hopf $(Γ, ω)$-algebra is constructed, which gives rise to a group functor corresponding to the general linear group in the $Γ$-graded setting. Using this Hopf $(Γ, ω)$-algebra, we realise simple tensor modules and their dual modules by mimicking the classic Borel-Weil theorem. We also analyse in some detail the case with $Γ={\mathbb Z}^{\dim{V(Γ, ω)}}$ and $ω$ depending on a complex parameter $q\ne 0$, where $\mathfrak{gl}(V(Γ, ω))$ shares common features with the quantum general linear (super)group, but is better behaved especially when $q$ is a root of unity.