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# Another example of a projection matrix

Figuring out the transformation matrix for a projection onto a subspace by figuring out the matrix for the projection onto the subspace's orthogonal complement first. Created by Sal Khan.

Video transcript

Let's say I have a subspace v
that is equal to all of the vectors-- let me write it this
way-- all of the x1, x2, x3's, so all the vectors like this
that satisfy x1 plus x2 plus x3 is equal to 0. So if you think about it, this
is just a plane in R3, so this subspace is a plane in R3. And I'm interested in finding
the transformation matrix for the projection of any vector
x in R3 onto v. So how could we do that? So we could do it like we
did in the last video. We could find the basis for
this subspace right there. And that's not too hard to do. We could say x1, if we assume
that, let's say, that x2 and x3 are kind of free variables,
then we could say that x1 is equal to minus x2, minus x3. And then let's just, just so we
can write it in, kind of, our parametric form, or if we
can write our solution set as the combination of basis
vectors, we can say x2 is equal to, let's say
it's equal to some arbitrary constant, C2. And let's say that x3
is equal to some arbitrary constant, C3. Then we can say that v, we can
rewrite v, we could say that v is-- I'll do it here-- v is
equal to the set of all x1's, x2's, and x3's that are equal
to C2 times-- so x1 is equal to minus-- let me rewrite
this with a C2-- this is equal to C2. This is equal to C3. So x1 is equal to minus
C2, minus C3. So x1 is equal to minus 1 times
C2, plus C3 times what? Plus C3 times minus 1. And then what is x2 equal to? x2 is just equal to C2. So it's 1 times C2,
plus 0, times C3. x3 is just equal to C3,
so it's 0 times C2, plus 1, times C3. And so this is another way
of defining our subspace. All of the vectors that satisfy
this is equal to this definition here. It's all the vectors whose
components satisfy, or that lie in this plane, whose entries
lie in that plane. And that's for any real
numbers right there. Or another way of writing this,
is v is equal to the span of the vectors minus 1, 1,
and 0, and the vector minus 1, 0, and 1. Just like that. And we know that these are
actually a basis for v because they're linearly independent. There's no way I can take linear
combinations of this guy and make the second
entry equal a 1 here. And likewise there's no way I
can take linear combinations of this guy and make this third
entry equal a 1 here. So these are also
a basis for v. So given, that just using the
technique we did before, we could set some vector, we could
set some matrix A equal to minus 1, 1, 0, and then
minus 1, 0, and 1. And then we can figure out that
the projection of any vector x in our 3 onto v is
going to be equal to, and we saw this, it's going to be equal
to A times the inverse of A transpose A. All of that times A transpose
and all of that times x. And you can do it. You have A here. You can figure out what the
transpose of A is very easy. You can take A transpose A,
then you can invert it. And it'll be very similar to
what we did in the last video. It'll be a little less work,
because this is a 3 by 2 matrix, instead of
a 4 by 2 matrix. But you saw it is actually
a lot of work. It's very hairy and you might
make some careless mistakes. So let's figure out if there's
another way that we can come up with this matrix
right here. Now we know that if x is a
member of R3, that x can be represented as a combination of
some vector v, that is in our subspace, plus some vector
w, that is in the orthogonal complement of the subspace,
where v is a member of our subspace, and w is a member of
the orthogonal complement of our subspace. Now by definition, that right
there is the projection of x onto v, and this is the
projection of x onto the orthogonal complement of v. So we can write that x is equal
to the projection onto v of x, plus the projection onto
v's orthogonal complement, or the orthogonal complement
of v of x. So this is by definition, that
any member of R3 can be represented this way. Now if we want to write this as
matrix vector products, and two videos ago I showed you
that these are linear transformations. So let me write that here. So they're linear
transformations. So they can be written as
matrix vector products. You see that right there. Let me define this matrix, I
don't know, let me call this matrix T, let me
just call it T. And let me do another. Let me do a letter,
let me do B. And let's say that the
projection onto the orthogonal complement of v of x, let's say
that that's equal to some other matrix C, times x. And we know this is a linear
transformation, so it can be represented as some
matrix C times x. So what are these going
to be equal to? Well x, if I want to write it as
a linear transformation of x, I could just write it as the
3 by 3 identity matrix, times x, right? That's the same thing as x. That's going to be equal to the
projection of x onto v, well that's just the same
thing as B times x. And then plus the projection
of x onto v's orthogonal complement, well that's
just C times x. Plus C times x. And if you want to factor out
the x on this side, we know that the matrix vector
products exhibit the distributive property, so we
could write that the identity matrix times x is equal
to B plus C, times x. Or another way to view this
equation is that this matrix must be equal to these
two matrices. So we get that the identity
matrix in R3 is equal to the projection matrix onto v, plus
the projection matrix onto v's orthogonal complement. Remember, the whole point of
this problem is to figure out this thing right here,
is to solve or B. And we know a technique
for doing it. You take A transpose, you can do
this whole thing, but that might be pretty hairy. But maybe it's easy
to find this guy. Maybe, I don't know. It actually turns out in the
video, this one will be easy. So if it's easy to find this
guy, we can just solve for B. If we subtract C from both
sides, we get that B is equal to I, is equal to the identity
matrix, minus the transformation matrix for the
transformation onto v's orthogonal complement. So let's see what this is. Let's see if we can figure out
what C is right there. So let's go back to
our original. So remember-- let me rewrite
the problem actually-- remember that v was equal to,
essentially it's equal to all of the x1's, x2's, x3's that
satisfy x1 plus x2, plus x3 is equal to 0. Or another way to say it is that
all the x1's, x2's, and x3's that satisfy the equation
1, 1, 1, times x1, x2, x3 is equal to the 0 vector. Or this case it'll just be 0. We could write the 0 vector
just like that. So 1 times x1, plus 1 times x2,
plus 1 times x3 is going to equal the 0 vector. This is another way
to write v. Now all of the x's that
satisfied this right here, what is that? This is saying that v is equal
to the null space of this matrix right there. The null space of this matrix
is all of the vectors that satisfy this equation. So v is equal to the null
space-- let me write it this way-- the null space of 1,
1, 1, just like that. Up here we, kind of, figured
out v in kind of the traditional way. We figured out that v is the
span of these things, but now we know that's the same thing is
the null space of 1, 1, 1. These two statements
are equivalent. Now we at least had a hunch that
maybe, you know, we could figure out, straight up, this B
here by doing all of this A transpose and, you know,
by doing all of this silliness here. But our hunch is maybe if
we could figure out the transformation matrix for the
orthogonal complement of v right there, that then we could
just apply this, kind of, that we can just solve for
B given that the identity matrix minus this guy is
going to be equal to B. So let's see if we can figure
out the projection matrix, if we can figure out the
transformation matrix for the orthogonal projection, for
x onto the orthogonal projection of v. So this is v. What is v compliment? v compliment is going to be
equal to the orthogonal complement, or v perp is going
to be equal to the orthogonal complement of the null space
of this matrix right here. Which is equal to what? Remember, the null space, its
orthogonal complement-- a null space's orthogonal complement is
equivalent to the row space or the column space
of A transpose. We saw that multiple times. Or you could say the orthogonal
complement of the row space is the null space. We've seen this many,
many times before. So the orthogonal complement of
this guy is going to be the column space of his transpose. So the column space of the
transpose of this guy. So it's 1, 1, 1,
just like that. Or we can write that v's
orthogonal complement is equal to the span of 1, 1, 1. The column space of this matrix,
we only have one column in it, so its column
space is going to be the span of that one column. So just to visualize what
we're doing here, that original equation for v, that
satisfies that, that's just going to be some plane in R3. That is v right there. And now we just figured
out what v's orthogonal complement is. It's going to be a line in R3. It's going to be all
of the linear combinations of this guy. So it's going to be
some line in R3. I haven't drawn it. You know this is going to be
tilted more, and so is this, but it's going to
be some line. So this is the orthogonal
complement of v. So let's see if we
can figure out. So remember, the projection--
let me do it this way. So this is the basis for v's
orthogonal complement. So let's construct
some matrix. I don't know, let me
use a letter that I haven't used before. Let me construct some matrix D,
whose columns are the basis vectors for the orthogonal
complement of v. Well, there's only one
basis vector, so it's going to be that. And we learned, in the last
video and the video before that, that the projection of
any vector in R3 onto v's orthogonal complement is going
to be equal to D times D transpose D inverse, times
D transpose, times x. Or another way to view it is
that this thing right here, that thing right there is the
transformation matrix for this projection. That is the transformation
matrix. matrix So let's see if this is easier
to solve this thing than this business up here, where we
had a 3 by 2 matrix. That was the whole motivation
for doing this problem. To figure out the projection
matrix for v's subspace, we'd have to do this with
the 3 by 2 matrix. It seems pretty difficult. Instead, let's find the
projection matrix to get to the production onto
v's orthogonal complement, which is this. So what is D transpose? So D transpose is just going
to be equal to 1, 1, 1. What is D transpose times D? Well, that's D transpose. This is D, just like that. So what is this going
to be equal to? This is just the dot product
of that and that. 1 times 1, plus 1 times 1, plus
1 times 1, it equals 3. So this thing right here is
equal to a 1 by 1 matrix 3. So let's write it down. So this is equal to D-- which
is this matrix, 1, 1, 1-- times D transpose D inverse. So D transpose D is just
a 1 by 1 matrix. And we're going to have
to invert it. Actually, I've never defined the
inverse of a 1 by 1 matrix for you just now, so it's
just mildly exciting. Times D transpose. So D transpose looks
like this, 1, 1, 1. And then all of that's
times x. But this is the transformation
matrix right there. Now what is the inverse
of a 1 by 1 matrix? Now you just have to remember
that A inverse times A is equal to the identity matrix. If we're dealing with a 1 by 1
matrix, then I'm just trying to figure out what, let's say,
what matrix times 3 is going to be equal to the 1 by
1 identity matrix. So let's say that 3 inverse
times 3 has to be equal to the identity matrix. 1 by 1 identity matrix. Well, the only matrix that's
going to make this work out, to get this entry I'll just take
this guy's entry times that guy's entry, is going to
be this guy right here. The inverse of this 1
by 1 matrix has to be the matrix 1/3. 1/3 times 3 is equal to 1. This is almost trivially
simple, but this is the inverse, that right there is the
inverse matrix, for the 1 by 1 matrix 3. So this right here
is just 1/3. And we could actually
just take that out. It's a 1 by 1 matrix,
which is essentially equivalent to a scalar. So this is going to be equal--
let me just draw a line here-- this thing is equal to 1/3--
actually, I don't want to confuse you. Let me rewrite it. So we get the projection of
any vector in R3 onto the orthogonal complement of v, is
equal to 1/3, that's 1/3, times the vector 1, 1, 1,
times-- sorry, or wait, that is a vector or the matrix 1
on 1-- times that matrix transposed, 1, 1, 1. And then all of that times x. And you can see, this is a lot
simpler than if we have to do all of this business
with this matrix. That's a harder matrix
to deal with. This 1, 1, 1 matrix
is very easy. Now what is this going
to be equal to? This is going to be equal to
1/3 times, we have a 3 by 1 times a 1 by 3 matrix,
so it's going to result in a 3 by 3 matrix. And what do we get? So this first entry is going to
be 1 times 1, which is 1. The second entry is going to
be 1 times 1, which is 1. The third entry is going to
be 1 times 1, which is 1. I think you see the pattern. The second grow, first column,
1 times 1, which is 1. So this is going to be a
3 by 3 matrix of 1's. So just like that we were able
to get-- that was a pretty straightforward situation--
we were able to get the projection matrix for any
vector in R3 onto v's orthogonal complement. Now, we know that this thing
right here is our original C that we said. And we said that the identity
matrix-- we wrote it up here. Let me refer back to what
I wrote way up here. We said, look, the identity
matrix is equal to the transformation matrix for the
projection onto v, plus the transformation matrix for the
projection onto v's orthogonal complement. Or we can write that the
transformation matrix for the projection onto v is equal to
the identity matrix minus the transformation matrix for the
projection onto v's orthogonal complement. So if we say that the projection
onto v of x is equal to B times x, we know that
B is equal to the 3 by 3 identity matrix, minus C, and
this is C right there. So B is equal to the identity
matrix-- so that's just 1, 0, 0, 0, 1, 0, 0, 0, 1-- minus C,
minus 1/3, times 1, 1, 1, 1, 1, 1, 1, 1, 1, just like that. And what is this going
to be equal to? Let's see, let's, in our heads,
multiply this out. All of these entries are going
to be 1/3 essentially, if we multiply this out like that. So if we have 1 minus 1/3. I could write it
out like that. It's 1/3, 1/3, 1/3. Everything is 1/3. 1/3, 1/3, 1/3, 1/3, 1/3, 1/3,
and this just becomes a 1. So 1 minus 1/3 is 2/3. And all of the 1's minus 1/3
are going to be 2/3, so we could just go down
the diagonal. And then the 0's minus 1/3 are
going to be minus 1/3. Minus 1/3, minus
1/3, minus 1/3. You have minus 1/3, minus
1/3, and minus 1/3. And just like that, we've been
able to figure out our projection, our transformation
matrix, for the projection of any vector x onto v, by
essentially finding this guy first, for finding the
transformation matrix for the projection of any x onto v's
orthogonal complement. Anyway, I thought that
was pretty neat. And you could rewrite this as v
equal to 1/3 times 2, 2, 2, 2's along back the diagonals
and then you have minus 1's everywhere else. Anyway, see you in
the next video.