Comparison of the simulated, solid black lines, and analytical, solid red lines, 1D B-modes kernels, , shown as a function of , and computed for the unlensed CMB E power contained initially only in a single mode, ℓ^E = 500,1000,4000 and 5000. For low values of ℓ^E, left panels, the agreement between the analytic expression, Eq. (12), and numerical results is very good all the way to ℓ^B ≲ 2000, as expected. For higher values of ℓ^E, though the agreement is poorer, it remains qualitatively very good, which justifies our semi-analytic considerations presented in Sect. 3.