  The Nowak modification of the Sznajd opinion dynamics model on the square
lattice assumes that with probabilities beta and gamma the opinions flip due to
mass-media advertising from down to up, and vice versa. Besides, with
probability alpha the Sznajd rule applies that a neighbour pair agreeing in its
two opinions convinces all its six neighbours of that opinion. Our Monte Carlo
simulations and mean-field theory find sharp phase transitions in the parameter
space.
