Today is going to be one of the
more difficult lectures of the
term.
So, put on your thinking caps,
as they would say in elementary
school.
The topic is going to be what's
called a convolution.
The convolution is something
very peculiar that you do to two
functions to get a third
function.
It has its own special symbol.
f of t asterisk is the
universal symbol that's used for
that.
So, this is a new function of
t, which bears very little
resemblance to the ones,
f of t, that you started with.
I'm going to give you the
formula for it,
but first, there are two ways
of motivating it,
and both are important.
There is a formal motivation,
which is why it's tucked into
the section on Laplace
transform.
And, the formal motivation is
the following.
Suppose we start with the
Laplace transform of those two
functions.
Now, the most natural question
to ask is, since Laplace
transforms are really a pain to
calculate is from old Laplace
transforms, is it easy to get
new ones?
And, the first thing,
of course, summing functions is
easy.
That gives you the sum of the
transforms.
But, a more natural question
would be, suppose I want to
multiply F of t and G
of t.
Is there, hopefully,
some neat formula?
If I multiply the product of
the, take the product of these
two, is there some neat formula
for the Laplace transform of
that product?
That would simply life greatly.
And, the answer is,
there is no such formula.
And there never will be.
Well, we will not give up
entirely.
Suppose we ask the other
question.
Suppose instead I multiply the
Laplace transforms.
Could that be related to
something I cook up out of F of
t and G of t?
Could it be the transform of
something I cook up out of F of
t or G of t?
And, that's what the
convolution is for.
The answer is that F of s times
G of s turns out
to be the Laplace transform of
the convolution.
The convolution,
and that's one way of defining
it, is the function of t you
should put it there in order
that its Laplace transform turn
out to be the product of F of s
times G of s.
Now, I'll give you,
in a moment,
the formula for it.
But, I'll give you one and a
quarter minutes,
well, two minutes of motivation
as to why there should be such
formula.
Now, I won't calculate this out
to the end because I don't have
time.
But, here's the reason why
there should be such formula.
And, you might suspect,
and therefore it would be worth
looking for.
It's because,
remember, I told you where the
Laplace transform came from,
that the Laplace transform was
the continuous analog of a power
series.
So, when you ask a general
question like that,
the place to look for is if you
know an analogous idea,
say, does it work.
something like that work there?
So, here I have a power series
summation, (a)n x to the n.
Remember, you can write this in
computer notation as a of n
to make it look like f
of n,
f of t.
And, the analog is turned into
t when you turn a power series
into the Laplace transform,
and x gets turned into e to the
negative s,
and one formula just turns into
the other.
Okay, so, there's a formula for
F of x.
This is the analog of the
Laplace transform.
And, similarly,
G of x here is
summation (b)n x to the n.
Now, again, the naīve question
would be, well,
suppose I multiply the two
corresponding coefficients
together, and add up that power
series, summation (a)n (b)n
times x to the n.
Is that somehow,
that sum related to F and G?
And, of course,
everybody knows the answer to
that is no.
It has no relation whatever.
But, suppose instead I multiply
these two guys.
In that case,
I'll get a new power series.
I don't know what its
coefficients are,
but let's write them down.
Let's just call them (c)n's.
So, what I'm asking is,
this corresponds to the product
of the two Laplace transforms.
And, what I want to know is,
is there a formula which says
that (c)n is equal to something
that can be calculated out of
the (a)i and the (b)j.
Now, the answer to that is,
yes, there is.
And, the formula for (c)n is
called the convolution.
Now, you could figure out this
formula yourself.
You figure it out.
Anyone who's smart enough to be
interested in the question in
the first place is smart enough
to figure out what that formula
is.
And, it will give you great
pleasure to see that it's just
like the formula for the
convolution of going to give you
now.
So, what is that formula for
the convolution?
Okay, hang on.
Now, you are not going to like
it.
But, you didn't like the
formula for the Laplace
transform, either.
You felt wiser,
grown-up getting it.
But it's a mouthful to swallow.
It's something you get used to
slowly.
And, you will get used to the
convolution equally slowly.
So, what is the convolution of
f of t and g of t?
It's a function calculated
according to the corresponding
formula.
It's a function of t.
It is the integral from zero to
t of f of u, --
u is a dummy variable because
it's going to be integrated out
when I do the integration,
g of (t minus u) dt.
That's it.
I didn't make it up.
I'm just varying the bad news.
Well, what do you do when you
see a formula?
Well, the first thing to do,
of course, is try calculating
just to get some feeling for
what kind of a thing,
you know.
Let's try some examples.
Let's see, let's calculate,
what would be a modest
beginning?
Let's calculate the convolution
of t with itself.
Or, better yet,
let's calculate the convolution
just so that you could tell the
difference, t with t squared,
t squared with t,
to make it a little easier.
By the way, the convolution is
symmetric.
f star g is the same thing as g
star f.
Let's put that down explicitly.
I forgot to last period.
So, tell all the guys who came
to the one o'clock lecture that
you know something that they
don't.
Now, that's a theory.
It's commutative.
This operation is commutative,
in other words.
Now, that has to be a theorem
because the formula is not
symmetric.
The formula does not treat f
and g equally.
And therefore,
this is not obvious.
It's at least not obvious if
you look at it that way,
but it is obvious if you look
at it that way.
Why?
In other words,
f star g is the guy
whose Laplace transform is F of
s times G of s.
Well, what would g star f?
That would be the guy whose
Laplace transform is G
times F.
But capital F times capital G
is the same as capital G times
capital F.
So, it's because the Laplace
transforms are commutative.
Ordinary multiplication is
commutative.
It follows that this has to be
commutative, too.
So, I'll write that down,
since F times G is equal to GF.
And, you have to understand
that here, I mean that these are
the Laplace transforms of those
guys.
But, it's not obvious from the
formula.
Okay, let's calculate the
Laplace transform of,
sorry, the convolution of t
star,
let's do it by the formula.
All right, by the formula,
I calculate integral zero to t.
Now, I take the first function,
but I change its variable to
the dummy variable,
u.
So, that's u squared.
I take the second function and
replace its variable by u minus
t.
So, this is times t minus u,
sorry.
Okay, do you see that to
calculate this is what I have to
write down?
That's what the formula
becomes.
Anything wrong?
Oh, sorry, the du,
the integration's with expect
to u, of course.
Thanks very much.
Okay, let's do it.
So, it is, integral of u
squared t is,
remember, it's integrated with
respect to u.
So, it's u cubed over three
times t.
The rest of it is the integral
of u cubed,
which is u to the forth over
four.
All this is to be evaluated
between zero and t at the upper
limit.
So, I put u equal t,
I get t to the forth over three
minus t to the forth
over four.
Of course, at the lower limit,
u is zero.
So, both of these are terms of
zero.
There's nothing there.
And, the answer is,
therefore, t to the forth
divided by,
a third minus a quarter is a
twelfth.
So, that's doing it from the
formula.
But, of course,
there is an easier way to do
it.
We can cheat and use the
Laplace transform instead.
If I Laplace transform it,
the Laplace transform of t
squared is what?
It's two factorial divided by s
cubed.
The Laplace transform of t
is one divided by s
squared.
And so, because this is the
convolution of these,
it should correspond to the
product of the Laplace
transforms, which is two over s
to the 5th power.
Well, is that the same as this?
What's the Laplace transform
of, in other words,
what's the inverse Laplace
transform of two over s to the
fifth?
Well, the inverse Laplace
transform of four factorial over
s to the fifth is how much?
That's t to the forth,
right?
Now, how does this differ?
Well, to turn that into that,
I should divide by four times
three.
So, this should be one twelfth
t to the forth,
one over four times three
because this is 24,
and that's two,
so, divide by 12 to determine
what constant,
yeah.
So, it works,
at least in that case.
But now, notice that this is
not an ordinary product.
The convolution of t squared
and t is not something
like t cubed.
It's something like t to the
forth, and there's a funny
constant in there,
too, very unpredictable.
Let's look at the convolution.
Let's take another example of
the convolution.
Let's do something really
humble just assure you that
this, even at the simplest
example, this is not trivial.
Let's take the convolution of f
of t with one.
Can you take,
yeah, one is a function just
like any function.
But, you get something out of
the convolution,
yes, yes.
Let's just write down the
formula.
Now, I can't use the Laplace
transform here because you won't
know what to do with it.
You don't have that formula
yet.
It's a secret one that only I
know.
So, let's do it.
Let's calculate it out the way
it was supposed to.
So, it's the integral from zero
to t of f of u,
and now, what do I do with that
one?
I'm supposed to take,
one is the function g of t,
and wherever I see a
t, I'm supposed to plug in t
minus u.
Well, I don't see any t there.
But that's something for
rejoicing.
There's nothing to do to make
the substitution.
It's just one.
So, the answer is,
it's this curious thing.
The convolution of a function
with one, you integrate it from
zero to t.
Well, as they said in Alice in
Wonderland, things are getting
curiouser and curiouser.
I mean, what is going on with
this crazy function,
and where are we supposed to
start with it?
Well, I'm going to prove this
for you, mostly because the
proof is easy.
In other words,
I'm going to prove that that's
true.
And, as I give the proof,
you'll see where the
convolution is coming from.
That's number one.
And, number two,
the real reason I'm giving you
the proof: because it's a
marvelous exercise in changing
the variables in a double
integral.
Now, that's something you all
know how to do,
even the ones who are taking
18.02 concurrently,
and I didn't advise you to do
that.
But, I've arranged the course
so it's possible to do.
But, I knew that by the time we
got to this, you would already
know how to change variables at
a double integral.
So, and in fact,
you will have the advantage of
remembering how to do it because
you just had it about a week or
two ago, whereas all the other
guys, it's something dim in
their distance.
So, I'm reviewing how to change
variables at a double integral.
I'm showing you it's good for
something.
So, what we are out to try to
prove is this formula.
Let's put that down in,
so you understand.
Okay, let's do it.
Now, we'll use the desert
island method.
So, you have as much time as
you want.
You're on a desert island.
In fact, I'm going to even go
it the opposite way.
I'm going to start with--
you've got a lot of time on your
hands and say,
gee, I wonder if I take the
product of the Laplace
transforms, I wonder if there's
some crazy function I could put
in there, which would make
things work.
You've never heard of the
convolution.
You're going to discover it all
by yourself.
Okay, so how do you begin?
So, we'll start with the left
hand side.
We're looking for some nice way
of calculating that as the
Laplace transform of a single
function.
So, the way to begin is by
writing out the definitions.
We couldn't use anything else
since we don't have anything
else to use.
Now, looking ahead,
I'm going to not use t.
I'm going to use two neutral
variables when I calculate.
After all, the t is just a
dummy variable anyway.
You will see in a minute the
wisdom of doing this.
So, it's this times the
integral, which gives the
Laplace transform of g.
So, that's e to the negative s
v, let's say,
times g of v,
dv.
Okay, everybody can get that
far.
But now we have to start
looking.
Well, this is a single
integral, an 18.01 integral
involving u, and this is an
18.01 integral involving v.
But when you take the product
of two integrals like that,
remember when you evaluate a
double integral,
there's an easy case where it's
much easier than any other case.
If you could write the inside,
if you are integrating over a
rectangle, for example,
and you can write the integral
as a product of a function just
of u, and a product of a
function just as v,
then the integral is very easy
to evaluate.
You can forget all the rules.
You just take all the u part
out, all the v part out,
and integrate them separately,
a to b, c to d.
That's the easy case of
evaluating a double integral.
It's what everybody tries to
do, even when it's not
appropriate.
Now, here it is appropriate,
except I'm going to use it
backwards.
This is the result of having
done that.
If this is the result of having
done it, what was the step just
before it?
Well, I must have been trying
to evaluate a double integral as
u runs from zero to infinity and
v runs from zero to infinity,
of what?
Well, of the product of these
two functions.
Now, what is that?
e to the minus s u times e to
the minus s v.
Well, I must surely want to
combine those.
e to the minus s u times e to
the minus s v.
And, what's left?
Well, what gets dragged along?
du dv.
This is the same as that
because of that law I just gave
you this is the product of a
function just of u,
and a function just of v.
And therefore,
it's okay to separate the two
integrals out that way because
I'm integrating sort of a
rectangle that goes to infinity
that way and infinity that way.
But, what I'm integrating is
over the plane,
in other words,
this region of the plane as u,
v goes from zero to infinity,
zero to infinity.
Now, let's take a look.
What are we looking for?
Well, we're looking for,
we would be very happy if u
plus v were t.
Let's make it t.
In other words,
I'm introducing a new variable,
t, u plus v,
and it's suggested by the form
in which I'm looking for the
answer.
Now, of course you then have
to, we need another variable.
We could keep either u or v.
Let's keep u.
That means v,
we just gave a musical chairs.
v got dropped out.
Well, we can't have three
variables.
We only have room for two.
But, we will remember it.
Rest in peace,
v was equal to t minus u
in case we ever need him
again.
Okay, let's now put in the
limits.
Let's put in the integral,
the rest of the change of
variable.
So, I'm now changing it to
these new variables,
t and u, so it's e to the
negative s t.
Well, f of u I don't
have to do anything to.
But, g of v,
I'm not allowed to keep v,
so v has to be changed to t
minus u.
Amazing things are happening.
Now, I want to change this to
an integral du dt.
Now, for that,
you have to be a little
careful.
We have two things to do to
figure out this;
what goes with that?
And, we have to put in the
limits, also.
Now, those are the two
nontrivial operations,
when you change variables in a
double integral.
So, let's be really careful.
Let's do the easier of the two,
first.
I want to change from du dv to
du dt.
And now, to do that,
you have to put in the Jacobian
matrix, the Jacobian
determinant.
Ah-ha!
How many of you forgot that?
I won't even bother asking.
Oh, come on,
you only lose two points.
It doesn't matter if you put it
in the Jacobian.
As you see, you're going to
forget something.
You will lose less credit for
forgetting than anything else.
So, it's the Jacobian of u and
v with respect to u and t.
So, to calculate that,
you write u equals u,
v equals t minus u,
and then the Jacobian is
the partial of the matrix,
the determinant of partial
derivatives.
So, it's the determinant whose
entries are the partial of u
with respect to u,
the partial of u with respect
to t, but these are independent
variables.
So, that's zero.
The partial of v with respect
to u is negative one.
The partial of v with respect
to t is one.
So, the Jacobian is one.
So, if you forgot it,
no harm.
So, the Jacobian is one.
Now, more serious,
and in some ways,
I think, for most of you,
the most difficult part of the
operation, is putting in the new
limits.
Now, for that,
you look at the region over
which you're integrating.
I think I'd better do that
carefully.
I need a bigger picture.
That's really what I'm trying
to say.
So, here's the (u,
v) coordinates.
And, I want to change these to
(u, t) coordinates.
The integration is over the
first quadrant.
So, what you do is,
when you do the integral,
the first step is u is varying,
and t is held fixed.
So, in the first integration,
u varies.
t is held fixed.
Now, what is holding t fixed in
this picture mean?
Well, t is equal to u plus v.
So, u plus v is fixed,
is a constant,
in other words.
Now, where are the curves along
which u plus v is a
constant?
Well, they are these lines.
These are the lines along which
u plus v equals a constant,
or t is a constant.
The reason I'm holding t a
constant is because the first
integration only allows u to
change.
t is held fixed.
Okay, you let u increase.
As u increases,
and t is held fixed,
I'm traversing these lines in
this direction.
That's the direction on which u
is increasing.
I integrate from the point,
from the u value where they
leave the region.
And, to enter the region,
what's the u value where they
enter the region?
u is equal to zero.
Everybody would know that.
Not so many people would be
able to figure out what to put
for where it leaves the region.
What's the value of u when it
leaves the region?
Well, this is the curve,
v equals zero.
But, v equals zero is,
in another language,
u equals t.
t minus u equals zero,
or u equals t.
In other words,
they enter the region where u
equals zero,
and they leave where u is t,
has the value of t.
And, how about the other guys?
For which t's do I want to do
this?
Well, I want to do it for all
these t values.
Well, now, the t value here,
that's the starting one.
Here, t is zero,
and here t is not zero.
And, if I go out and cover the
whole first quadrant,
I'll be letting t increase to
infinity.
The sum of u and v,
I will be letting increase to
infinity.
So, it's zero to infinity.
So, all this is an exercise in
taking this double integral in
(u, v) coordinates,
and changing it to this double
integral, an equivalent double
integral over the same region,
but now in (u,
t) coordinates.
And now, that's the answer.
Somewhere here is the answer
because, look,
since the first integration is
with respect to u,
this guy can migrate outside
because it doesn't involve u.
That only involves t,
and t is only caught by the
second integration.
So, I can put this outside.
And, what do I end up with?
The integral from zero to
infinity of e to the negative s
t times,
what's left?
A funny expression,
but you're on your desert
island and found it.
This funny expression,
integral from zero to t,
f of u, g of t minus u vu,
in short,
the convolution,
exactly the convolution.
So, all you have to do is get
the idea that there might be a
formula, sit down,
change variables and double
integral it, ego,
you've got your formula.
Well, I would like to spend
much of the rest of the
period--- in other words,
that's how it relates to the
Laplace transform.
That's how it comes out of the
Laplace transform.
Here's how to use it,
calculate it either with the
Laplace transform or directly
from the integral.
And, of course,
you will solve problems,
Laplace transform problems,
differential equations using
the convolution.
But, I have to tell you that
most people, convolution is very
important.
And, most people who use it
don't use it in connection with
the Laplace transform.
They use it for its own sake.
The first place I learned that
outside of MIT people used a
convolution was actually from my
daughter.
She's an environmental
engineer, an environmental
consultant.
She does risk assessment,
and stuff like that.
But anyway, she had this paper
on acid rain she was trying to
read for a client,
and she said something about
calculating acid rain falls on
soil.
And then, from there,
the stuff leeches into a river.
But, things happen to it on the
way.
Soil combines in various ways,
reduces the acidity,
and things happen.
Chemical reactions take place,
blah, blah, blah,
blah.
Anyways, she said,
well, then they calculated in
the end how much the river gets
polluted.
But, she said it's convolution.
She said, what's the
convolution?
So, I told her she was too
young to learn about the
convolution.
And she knows that I thought
I'd better look it up first.
I mean, I, of course,
knew the convolution was,
but I was a little puzzled at
that application of it.
So, I read the paper.
It was interesting.
And, in thinking about it,
other people have come to me,
some guy with a problem about,
they drilled ice cores in the
North Pole, and from the
radioactive carbon and so on,
deducing various things about
the climate 60 billion years
ago, and it was all convolution.
He asked me if I could explain
that to him.
So, let me give you sort of
all-purpose thing,
a simple all-purpose model,
which can be adapted,
which is very good way of
thinking of the convolution,
in my opinion.
It's a problem of radioactive
dumping.
It's in the notes,
by the way.
So, I'm just,
if you want to take a chance,
and just listen to what I'm
saying rather that just
scribbling everything down,
maybe you'll be able to figure
it out for the notes,
also.
So, the problem is we have some
factory, or a nuclear plant,
or some thing like that,
is producing radioactive waste,
not always at the same rate.
And then, it carts it,
dumps it on a pile somewhere.
So, radioactive waste is
dumped, and there's a dumping
function.
I'll call that f of t,
the dump rate.
That's the dumping rate.
Let's say t is in years.
You like to have units,
and quantity,
kilograms, I don't know,
whatever you want.
Now, what does the dumping rate
mean?
The dumping rate means that if
I have two times that are close
together, for example,
two successive days,
midnight on two successive
days, then there's a time
interval between them.
I'll call that delta t.
To say the dumping rate is f of
t means that the amount
dumped in this time interval,
in the time interval from t1 to
t1 plus one is
approximately,
not exactly,
because the dumping rate isn't
even constant within this time
interval.
But it's approximately the
dumping rate times the time over
which the dumping is taking
place.
That's what I mean by the dump
rate.
And, it gets more and more
accurate, the smaller the time
interval you take.
Okay, now here's my problem.
The problem is,
you start dumping at time t
equals zero.
At time t equal t,
how much radioactive waste is
in the pile?
Now, what makes that problem
slightly complicated is
radioactive waste decays.
If I put some at a certain day,
and then go back several months
later and nothing's happened in
between, I don't have the same
amount that I dumps because a
fraction of it decayed.
I have less.
And, our answer to the problem
must take account of,
for each piece of waste,
how long it has been in the
pile because that takes account
of how long it had to decay,
and what it ends up as.
So, the calculation,
the essential part of the
calculation will be that if you
have an initial amount of this
substance, and it decays for a
time, t, this is the amount left
at time t.
This is the law of radioactive
decay.
You knew that coming into
18.03, although,
it's, of course,
a simple differential equation
which produces it,
but I'll assume you simply know
the answer.
k depends on the material,
so I'm going to assume that the
nuclear plant dumps the same
radioactive substance each time.
It's only one substance I'm
calculating, and k is it.
So, assume the k is fixed.
I don't have to change from one
k from one material to a k for
another because it's mixing up
the stuff, just one material.
Okay, and now let's calculate
it.
Here's the idea.
I'll take the t-axis,
but now I'm going to change its
name to the u-axis.
You will see why in just a
second.
It starts at zero.
I'm interested in what's
happening at the time,
t.
How much is left at time t?
So, I'm going to divide up the
interval from zero to t on this
time axis into,
well, here's u0,
the starting point,
u1, u2, let's make this u1.
Oh, curses!
u1, u2, u3, and so on.
Let's call this (u)n.
So they're u(n + 1),
not that it matters.
It doesn't matter.
Okay, now, the amount,
so, what I'm going to do is
look at the amount,
take the time interval from ui
to ui plus one.
This is a time interval,
delta u.
Divide it up into equal time
intervals.
So, the amount dumped in the
time interval from u(i) to u(i
plus one)
is equal to approximately f of
u(i),
the dumping function there,
times delta u.
We calculated that before.
That's what the meaning of the
dumping rate is.
By time t, how much has it
decayed to?
It has decayed.
How much is left,
in other words?
Well, this is the starting
amount.
So, the answer is going to be
it's f of (u)i times delta u
times this factor,
which tells how much it decays,
so, time.
So, this is the starting amount
at time (u)i.
That's when it was first
dumped, and this is the amount
that was dumped,
times, multiply that by e to
the minus k times,
now, what should I put up in
there?
I have to put the length of
time that it had to decay.
What is the length of time that
it had to decay?
It was dumped at u(i).
I'm looking at time,
t, it decayed for time length t
minus u i,
the length of time it had all
the pile.
So, the stuff that was dumped
in this time interval,
at time t when I come to look
at it, this is how much of it is
left.
And now, all I have to do is
add up that quantity for this
time, the stuff that was dumped
in this time interval plus the
stuff dumped in,
and so on, all the way up to
the stuff that was dumped
yesterday.
And, the answer will be the
total amount left at time,
t, that is not yet decayed will
be approximately,
you add up the amount coming
from the first time interval
plus the amount coming,
and so on.
So, it will be f of u(i),
I'll save the delta u for the
end, times e to the minus k
times t minus u(i) times
delta u.
So, these two parts represent
the amount dumped,
and this is the decay factor.
And, I had those up as I runs
from, well, where did I start?
From one to n,
let's say.
And now, let delta t go to
zero, in other words,
make this delta u go to zero,
make this more accurate by
taking finer and finer
subdivisions.
In other words,
instead of looking every month
to see how much was dumped,
let's look every week,
every day, and so on,
to make this calculation more
accurate.
And, the answer is,
this approach is the exact
amount, which will be the
integral.
This sum is a Riemann sum.
It approaches the integral from
zero to, well,
I'm adding it up from u1 equals
zero to un equals t,
the final value.
So, it will be the integral
from the starting point to the
ending point of f of u e to the
minus k times t minus u to u.
That's the answer to the
problem.
It's given by this rather funny
looking integral.
But, from this point of view,
it's entirely natural.
It's a combination of the
dumping function.
This doesn't care what the
material was.
It only wants to know how much
was put on everyday.
And, this part,
which doesn't care how much was
put on each day,
it just is an intrinsic
constant of the material
involving its decay rate.
And, this total thing
represents the total amount.
And that is,
what is it?
It's the convolution of f of t
with what function?
e to the minus k t.
It's the convolution of the
dumping function and the decay
function.
And, the convolution is exactly
the operation that you have to
have to do that.
Okay, so, I think this is the
most intuitive physical approach
to the meaning of the
convolution.
In this particular,
you can say,
well, that's very special.
Okay, so it tells you what the
meaning of the convolution with
an exponential is.
But, what about the convolution
with all the other functions
we're going to have to use in
this course.
They can all be interpreted
just by being a little flexible
in your approach.
I'll give you two examples of
this, well, three.
First of all,
I'll use it for,
in the problem set I ask you
about a bank account.
That's not something any of you
are interested in.
Okay, so, suppose instead I
dumped garbage --
-- undecaying.
So, something that doesn't
decay at all,
what's the answer going to be?
Well, the calculation will be
exactly the same.
It will be the convolution of
the dumping function.
The only difference is that now
the garbage isn't going to
decay.
So, no matter how long it's
left, the same amount is going
to be left at the end.
In other words,
I don't want to exponential
decay function.
I want to function,
one, the constant function,
one, because once I stick it on
the pile, nothing happens to it.
It just stays there.
So, it's going to be the
convolution of this one because
this is constant.
It's undecaying --
-- by the identical reasoning.
And so, what's the answer going
to be?
It's going to be the integral
from zero to t of f of u du.
Now, that's an 18.01 problem.
If I dump with a dumping rate,
f of u,
and I dump from time zero to
time t, how much is on the pile?
They don't give it.
They always give velocity
problems, and problems of how to
slice up bread loaves,
and stuff like that.
But, this is a real life
problem.
If that's the dumping rate,
and you dump for t days from
zero to time t,
how much do you have left at
the end?
Answer: the integral of f of u
du from zero to t.
I'll give you another example.
Suppose I wanted a dumping
function, suppose I wanted a
function, wanted to interpret
something which grows like t,
for instance.
All I want is a physical
interpretation.
Well, I have to think,
I'm making a pile of something,
a metaphorical pile,
we don't actually have to make
a physical pile.
And, the thing should be
growing like t.
Well, what grows like t?
Not bacteria,
they grow exponentially.
Before the lecture,
I was trying to think of
something.
So, I came up with chickens on
a chicken farm.
Little baby chickens grow
linearly.
All little animals,
anyway, I've observed that
babies grow linearly,
at least for a while,
thank God.
After a while,
they taper off.
But, at the beginning,
they eat every four hours or
whatever.
And they eat the same amount,
pretty much.
And, that adds up.
So, let's suppose this
represents the linear growth of
chickens, of baby chicks.
That makes them sound cuter,
less offensive.
Okay, so, a farmer,
chicken farmer,
whatever they call them,
is starting a new brood.
So anyway, the hens lay at a
certain rate,
and each of those are
incubated.
And after a while,
little baby chicks come out.
So, this will be the production
rate for new chickens.
Okay, and it will be the
convolution which will tell you
at time, t, the number of
kilograms.
We'd better do this in
kilograms, I'm afraid.
Now, that's not as heartless as
it seems.
The number of kilograms of
chickens times t.
[LAUGHTER] It really isn't
heartless because,
after all, why would the farmer
want to know that?
Well, because a certain number
of pounds of chicken eat a
certain number of pounds of
chicken feed,
and that's how much he has to
dump, must have to give them
every day.
That's how he calculates his
expenses.
So, he will have to know the
convolution is,
or better yet,
he will hire you,
who knows what the convolution
is.
And you'll be able to tell him.
Okay, why don't we stop there
and go to recitation tomorrow.
I'll be doing important things.
