Weak Corrections to Three - Jet Production in Electron - Positron Annihilations : 1 ) The Factorisable

We report on the calculation of the factorisable one-loop weak-interaction corrections to the initial and final states for three-jet observables in electron-positron annihilations. We show that such corrections are of a few percent at √ s = MZ . Hence, while their impact is not dramatic in the context of LEP1 and SLC, where the total error on the measured value of αS is larger, at a future Linear Collider, running at the Z mass peak (e.g., GigaZ), they ought to be taken into account in the experimental fits, as here the uncertainty on the value of the strong coupling constant is expected to be at the 0.1% level or even smaller. The calculation has been performed using helicity amplitudes so that it can be applied to the case of polarised beams.