Abstract
We construct integrals of motion (IM) for the sine–Gordon model with boundary at the free fermion (β2=4π) which correctly determine the boundary S matrix. The algebra of these IM ("boundary quantum group" at q=1) is a one-parameter family of infinite-dimensional subalgebras of twisted [Formula: see text]. We also propose the structure of the fractional-spin IM away from the free fermion point (β2≠4π).
