Problem:
a) Solve the Helmholtz equation when u is a function of r only in 2-D.
b) Solve the Helmholtz equation when u is a function of r only in 3-D.
The Helmholtz equation
Consider both cases when λ > 0 and λ < 0.
**HINT** In 3-D, let u = v/r. In 2-D look up Bessel's equation.