General linear gauges and amplitudes in open string field theory

A general class of gauges for open string field theory, for which the gauge fixing condition is given by linear equations with respect to string field, is investigated in detail. This class of gauges includes almost all known ones like Siegel gauge and its various extensions such as a-gauges proposed by the present authors as well as Schnabl gauge and linear b-gauges. A general form of propagators is determined and their common features are analyzed. A consistent procedure for calculating the amplitudes is given. Gauge independence of the on-shell physical amplitudes is explicitly shown.