 "Q" is an antiderivation.The BRST Lagrangian density

While the Lagrangian density is not BRST invariant, its integral over all of spacetime, the action, is.The operator "Q" is defined as

where formula_91 are the Faddeev–Popov ghosts and antighosts (fields with a negative ghost number), respectively, "Li" are the infinitesimal generators of the Lie group, and formula_92 are its structure constants.