p. 181, equation (2) readPAA¯=PA¯2 for PAA¯=PA¯ p. 184, fourteenth line of §4 read 2np for 2p.sixteenth line of §4 readN(np²,np²q²) for N(2np², np²q²/4)equation (14) read 2n−3 for 2n−8