Consider the one-dimensional heat equation given by
This equation has Green's function which satisfies
The Green's function is given by
or,
Therefore,
Now,
Hence,
And,
Note that the second derivative of is a delta function.
We can use this observation to arrive at a more general description of the
Green's function for a particular differential equation. That is, for a
th order linear differential operator we would want the th derivative of
to be like a delta function. Thus the th derivative of
should be like a Heaviside function and all lower derivatives should
be continuous.
In particular, consider the operator acting on such that
with
where . If we integrate this equation across the
point from to we get
This condition is called a Jump condition.
This suggests that the Green's function satisfying
in the sense of distributions has the properties that
- for all .
- is continuous at for .
- .
- must satisfy all appropriate homogeneous boundary conditions.
If the Green's function exists then
Let us consider a second order differentiable linear operator, i.e., on
with separated boundary conditions,
where and are boundary operators.
The differential equation
and its required continuity conditions are satisfied is
The first two terms above are to satisfy the boundary conditions while the
third terms gives us continuity.
The jump condition is that
where is the Wronskian.
Therefore,
For the heat equation
we can take
That gives us and we recover the same as before.
In the general case, the solution is
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