2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/186891We construct a $Q$-matrix for the eight-vertex model at roots of unity for crossing parameter $η=2mK/L$ with odd $L$, a case for which the existing constructions do not work. The new $Q$-matrix $\Q$ depends as usual on the spectral parameter and also on a free parameter $t$. For $t=0$ $\Q$ has the standard properties. For $t\neq 0$, however, it does not commute with the operator $S$ and not with itself for different values of the spectral parameter. We show that the six-vertex limit of $\Q(v,t=iK'/2)$ exists.10 pages section on quasiperiodicity added, typo corrected, published versionStatistical MechanicsHigh Energy Physics - Theorytext