Universality properties of the stationary states in the one-dimensional coagulation-diffusion model with external particle input

We investigate with the help of analytical and numerical methods the reaction A + A --> A on a one-dimensional lattice opened at one end and with an input of particles at the other end. We show that if the diffusion rates to the left and to the right are equal, for large x, the particle concentration c(x) behaves like A(s)x(-1) (x measures the distance to the input end). If the diffusion rate in the direction pointing away from the source is larger than the one corresponding to the opposite direction. the particle concentration behaves like A(a)x(-1/2). The constants A(s) and A(a), are independent of the input and the two coagulation rates. The universality of A(d) comes as a surprise, since in the asymmetric case the system has a massive spectrum.