Solve Exercise 8.6.3 using one-dimensional eigenfunctions

Chapter 8, Problem 8.6.4

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QUESTION:

Solve (using two-dimensional eigenfunctions) \(\nabla^{2} u=Q(r, \theta)\) inside a circle of radius a subject to the given boundary condition. In what situations are there solutions?

(a) \(u(a, \theta)=0\)

(b) \(\frac{\partial u}{\partial r}(a, \theta)=0\)

(c) \(u(a, \theta)=f(\theta)\)

(d) \(\frac{\partial u}{\partial r}(a, \theta)=g(\theta)\)

Questions & Answers

QUESTION:

Solve (using two-dimensional eigenfunctions) \(\nabla^{2} u=Q(r, \theta)\) inside a circle of radius a subject to the given boundary condition. In what situations are there solutions?

(a) \(u(a, \theta)=0\)

(b) \(\frac{\partial u}{\partial r}(a, \theta)=0\)

(c) \(u(a, \theta)=f(\theta)\)

(d) \(\frac{\partial u}{\partial r}(a, \theta)=g(\theta)\)

ANSWER:

Step 1 of 4

Consider

(a) To solve the equation when  Consider Then the two-dimensional eigenfunctions are the set of 

Where, by the boundary condition

Use the method of eigenfunction expansion, to get

Where,

And

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