James V. Herod*

Page maintained with additions by Evans M. Harrell, II, harrell@math.gatech.edu.

**SECTION 2.5 AN UNDERSTANDING OF THE EQUATION L(y) =
/delta(x - t).**

**
**

**
By now you should believe that except for arithmetic details, you can work any
of these problems. We have come to the place where we need to get this problem
into perspective. **

**
We know that the
requirements of Section 2.4 give the
Kronecker delta symbol,
**

**
**

**
for any vector v.
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Thus A G = Id in components becomes**

**
**

**
When trying to solve differential equations, we might hope to find
G(.,t) as a solution to the equation L(G(.,t)) = \delta(.,t). Some understanding
of this equation is in order for the right side is not a function in the
ordinary sense. As has already been pointed out, it is a "generalized
function". The analogy with the matrix problem is pretty close: The delta
function in essence gives the continuous coordinates of the identity
operator:
**

**
**

**
for any a,b with a < x < b. Recall that the integral is a sort of
continuous sum, so this is appropriate.**

**
**

**
We present here, not a proof, but an understanding
that
**

**
L(G(.,t))(x) = \delta(x - t).
**

(2.6)The ideas should be examined and re-examined in later courses as a theory for generalized functions is developed:

Suppose that

and that equation (2.6) holds. Intuition is a guide:

If one were asked to solve the equation L(y) = f, where L is a reasonable second order operator, in the context of a sophomore differential equations course, one would think of the variations of parameter formula. In that setting, and for second order problems with u0 and u1 linearly independent solutions of the homogeneous equation,

y(x) = C

_{0}(x) u_{0}(x) + C_{1}(x) u_{1}(x).

Here,

and

This suggests an interpretation for "solution" of the second order equation:

L(G(.,t))(x) = \delta(x,t).

Namely, G (.,t) is the continuous function given by

(a) G(x,t) = C

_{0}(x,t)u_{0}(x) + C_{1}(x,t)u_{1}(x)

where

and

As above, the distribution equations should have solution

THEOREM.If, for each t, G(.,t) is in M and L(G(.,t))(x) = d(x,t) then G satisfies the four equations of Section 2.4.Proof. We hope to recognize the four equations which we used to define G for second order problems as arising from the above requirements for G. Two of those equations come from asking that G(.,t) should satisfy the two boundary equations. One other, G(t

^{+},t) - G(t^{-},t) = 0, comes from the requirement that G(.,t) should be continuous. To derive the equation

G

_{x}(t^{+},t) - G_{x}(t^{-},t) = 1/a_{2}(t),we first compute G

_{x}(x,t).G

_{x}(x,t) =

= C_{0,x}(x,t) u_{0}(x) + C_{0}(x,t) u_{0}'(x) + C_{1,x}(x,t) u_{1}(x)+ C_{1}(x,t) u_{1}'(x)

= C_{0}(x,t) u_{0}'(x) + C_{1}(x,t) u_{1}'(x).This last equality follows from (b). To find

G

_{x}(t^{+},t) - G_{x}(t^{-},t) = [C_{0}(t^{+},t)-C_{0}(t^{-},t)] u_{0}'(t) + [C_{1}(t^{+},t)-C_{1}(t^{-},t)] u'1(t),we must evaluate

In a similar manner,

[C

_{1}(t^{+},t) - C_{1}(t^{-},t)] = u_{0}(t) /a_{2}(t) w(t).Hence,

Hence, the inverse of the differential operator L on the set M is obtained by finding the function G( ,t) in M which satisfies (2.6),

L(G(.,t))(x) = \delta(x-t).

QED(2.6)

EXERCISES2.5

1. Give a formal argument, by interchanging limits and integrals, why if G satisfies (2.6), then

solves

L(u) = f.

Notice that the integral is the same as <G(x,t),f(t)>, if the inner product is calculated with t as the integration variable.

2. Give a formal argument to show that d(x) is an even function, in the sense that d(-x) = d(x). (Use a change of variables.)

3. Let G

_{1}(x,t) and G_{2}(x,t) be two Green functions for the differential equationu''(x) - u(x) = f(x).

Since the boundary conditions have not been specified, there will be many Green function for the differential equation

u''(x) - u(x) = f(x), u(0) = u(1) = 0.

(a) Classify G(x,t) as an integral kernel as in Chapter I. Is it separable? Is it small (in either sense)?

(b) Discuss how one could solve the integral equation

with the methods of Chapter I. To what differential equation is it equivalent?

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