  We solve the nonlinear Poisson-Boltzmann equation for two parallel and likely
charged plates both inside a symmetric elecrolyte, and inside a 2 : 1
asymmetric electrolyte, in terms of Weierstrass elliptic functions. From these
solutions we derive the functional relation between the surface charge density,
the plate separation, and the pressure between plates. For the one plate
problem, we obtain exact expressions for the electrostatic potential and for
the renormalized surface charge density, both in symmetric and in asymmetric
electrolytes. For the two plate problems, we obtain new exact asymptotic
results in various regimes.
