# How does perturbation method guarantee its solution for the perturbed pde $\Delta u + \epsilon u^2 =0$

My question is quite simple: Suppose we are given a PDE of with a boundary condition $$\Delta u + u^2 =0$$ where $u=u(r,\theta), 0<r<1$ and $u(1,\theta) = \cos\theta$ with $0 \leq \theta \leq 2\pi$.

Then we may perturb the nonlinear term, getting $$\Delta u + \epsilon u^2 =0$$ and look for a solution $$u(r,\theta;\epsilon) = \sum_{n=0}^{\infty} \epsilon^n u_n.$$ As substituting this into the perturbed PDE and considering the coefficients of the powers of $\epsilon$, we can get $u_0,u_1,...$ and so on with some algebraic work.

Then my question:

1) Can we analytically prove (or disprove) that the constructed $u$ is indeed a solution of the original PDE when taking $\epsilon =1$?

1.1) If so, is there any PDE textbook that deals with this type of matter in a systematic manner?

2) If not, should we prove numerically? Then how?

2.1) And for this numerical treatment, any textbook?

3) In general, where can I learn perturbation method properly in a rigorous setting with analysis?

Note on my knowledge: I'm new to PDE, currently learning by a textbook by Farlow and I know measure theory and some basic functional analysis.

[Duplication: mathoverflow]

3. To indicate that the matter is complicated here, I point out that the nonlinear problem you are talking about has two real solutions, therefore the approach in Farlow will allow you to find only one of them. Moreover, if you replace your boundary condition with $g(\theta)=A\cos \theta$, then for $A>20.65$ nonlinear problem will have no real solutions, so the series of approximations will be misleading.
4. This means also that the statement that you can solve the nonlinear problem by using $\epsilon=1$ in Farlow's book is false. If the boundary condition is $g(\theta)=0$ then $\Delta u=0$ has unique trivial solution, and $\Delta u+u^2=0$ has two real solutions.
5. The examples which I cited are taken from the Russian translation of Farlow's book. At the point where Farlow writes about the "rocket" that will bring you to the solution of the nonlinear problem, the editor of the Russian translation made a comment: "This rocket, first, not for all the boundary conditions will fly to $\epsilon=1$ (where for some boundary condition the world of complex solutions starts), and, second, never fly to the second real solution if it exists".