Twisted Fusion Products and Quantum Twisted $Q$-Systems

We obtain a complete characterization of the space of matrix elements dual to the graded multiplicity space arising from fusion products of Kirillov–Reshetikhin modules over special twisted current algebras defined by Kus and Venkatesh, which generalizes the result of Ardonne and Kedem to the special twisted current algebras. We also prove the conjectural identity of $q$-graded fermionic sums by Hatayama et al. for the special twisted current algebras, from which we deduce that the graded tensor product multiplicities of the fusion products of Kirillov–Reshetikhin modules over special twisted current algebras are both given by the $q$-graded fermionic sums, and constant term evaluations of products of solutions of the quantum twisted $Q$-systems obtained by Di Francesco and Kedem.