  We introduce a model for a population on a lattice with diffusion and
birth/death according to 2A->3A and A->0 for a particle A. We find that the
model displays a phase transition from an active to an absorbing state which is
continuous in 1+1 dimensions and of first-order in higher dimensions in
agreement with the mean field equation. For the 1+1 dimensional case, we
examine the critical exponents and a scaling function for the survival
probability and show that it belongs to the universality class of directed
percolation. In higher dimensions, we look at the first-order phase transition
by plotting a histogram of the population density and use the presence of phase
coexistence to find an accurate value for the critical point in 2+1 dimensions.
