That is, PDEs of the general form A(x, y, u) ∂u(x, y) ∂x + B(x,
y, u) ∂u(x, y) ∂y = C(x, y, u), (1) for some A, B and C. To solve
such PDEs we first find characteristics, curves in the solution
space (x, y, u) parametrically given by (x(τ ), y(τ ), u(τ )),
which satisfy dx dτ = A(x, y, u), dy dτ = b(x, y, u), du dτ = C(x,
y, u). (2) We find solutions...