On the notion of gauge symmetries of generic Lagrangian field theory

Treating gauge theories in a general setting, one meets the following problems: (i) any Lagrangian possesses gauge symmetries which therefore should be separated into the trivial and non-trivial ones, (ii) there is no intrinsic definition of higher-stage gauge symmetries, (iii) gauge and higher-stage gauge symmetries need not form an algebra. We define gauge symmetries as those associated to the Noether identities. Generic Lagrangian theory of even and odd fields on an arbitrary smooth manifold is considered. Under certain conditions, its non-trivial Noether and higher-stage Noether identities are well defined by constructing the antifield Koszul--Tate complex. The inverse second Noether theorem associates to this complex the cochain sequence of ghosts whose ascent operator provides all non-trivial gauge and higher-stage gauge symmetries of Lagrangian theory. This ascent operator, called the gauge operator, is not nilpotent, unless gauge symmetries are abelian. We replace a condition that gauge symmetries form an algebra with that the gauge operator can be extended to a nilpotent BRST operator. The necessary conditions of such an extension are stated.