We start by changing the Laplacian operator in the 2-D heat equation from rectangular to cylindrical coordinates by the following definition:
By changing the coordinate system, we arrive at the following nonhomogeneous PDE for the heat equation:
We choose for the example the Robin boundary conditions and initial conditions as follows:
This means that a separation constant can be found that both sides will equal. Let's define it to be This yields:
and multiplying the other side by yields:
After defining another separation constant , it yields:
Multiplying the other side by R yields:
We now have separate differential equations for each variable.
The SLP for is a singular Bessel type, whose eigenvalues depends on and are non-negative solutions to the following equation:
and the eigenfunction is:
where is the Bessel function of the first kind of order .
Let's define the solution as an infinite sum:
With the initial condition:
where
The weight function in the inner product in integrals involving the Bessel functions. The Bessel functions are orthogonal relative to the "weighted" scalar product
Solving the non-homogeneous equation involves defining the following functions:
Substitute the new definitions into the non-homogeneous equations:
We will use the following substitutions in our equation above:
We can eliminate the derivatives by substituting:
From the linear independence of , it follows that:
This first-order ODE can be solved with the following integration factor:
Thus, the equation becomes:
We satisfy the initial condition: