Example of a BOSS quasar spectrum of redshift 2.91 (smoothed to the width of analysis pixels). The red and blue lines cover the forest region used in our analysis, 104.0 <λ_RF< 120.0 nm. This region is sandwiched between the quasar’s Lyβ and Lyα emission lines, respectively at 400.9 and 475.4 nm (restframe 102.572 and 121.567 nm). The blue line is the model of the continuum, C_q(λ); the red line is the product of the continuum and the mean transmission, , as calculated by the method described in Sect. 4.