Liouvillian integrability of rational vector fields: The case of algebraic extensions

As shown in a previous paper, whenever a rational vector field on ${\mathbb{C}}^n$, $n>2$, is Liouvillian integrable, then it admits a first integral obtained by two successive integrations from a one-form with coefficients in a finite algebraic extension $L$ of the rational function field $K$. In the present work we discuss and characterize exceptional vector fields in this class, for which – by definition – the choice $L=K$ is not possible. In particular we show that exceptional vector field exist, giving explicit constructions in dimension three.