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# Gram-Schmidt example with 3 basis vectors

Gram-Schmidt example with 3 basis vectors. Created by Sal Khan.

Video transcript

Let's do one more Gram-Schmidt
example. So let's say I have the subspace
V that is spanned by the vectors-- let's say we're
dealing in R4, so the first vector is 0, 0, 1, 1. The second vector
is 0, 1, 1, 0. And then a third vector-- so
it's a three-dimensional subspace of R4-- it's 1, 1,
0, 0, just like that, three-dimensional
subspace of R4. And what we want to do, we want
to find an orthonormal basis for V. So we want to substitute these
guys with three other vectors that are orthogonal with
respect to each other and have length 1. So we do the same drill
we've done before. We can say-- let's call this
guy v1, this guy is v2, and let's call this guy v3. So the first thing we want to
do is replace v1-- and I'm just picking this guy at random
because he was the first guy on the
left-hand side. I want to replace v1 with an
orthogonal version of v1. So let me call u1 is equal to--
well, let me just find out the length the v1. I don't think I have to explain
too much of the theory at this point. I just want to show
another example. So the length of v1 is equal
to the square root of 0 squared plus 0 squared plus 1
squared plus 1 squared, which equals the square root of 2. So let me define my new vector
u1 to be equal to 1 over the length of v1, 1 over the square
root of 2, times v1, times 0, 0, 1, 1. And just like that, the span
of v1, v2, v3, is the same thing is the span of
u1, v2, and v3. So this is my first thing
that I've normalized. So I can say that V is now
equal to the span of the vectors u1, v2, and v3. Because I can replace v1 with
this guy, because this guy is just a scaled-up version
of this guy. So I can definitely represent
him with him, so I can represent any linear combination
of these guys with any linear combination of
those guys right there. Now, we just did our
first vector. We just normalized this one. But we need to replace these
other vectors with vectors that are orthogonal to
this guy right here. So let's do v2 first. So let's
replace-- let's call it y2 is equal to v2 minus the projection
of v2 onto the space spanned by u1 or onto--
you know, I could call it c times u1, or in the past
videos, we called that subspace V1, but the space
spanned by u1. And that's just going to be
equal to y2 is equal to v2, which is 0, 1, 1, 0, minus-- v2
projected onto that space is just a dot product of v2, 0,
1, 1, 0, with the spanning vector of that space. And there's only one of them,
so we're only going to have one term like this with u1, so
dotted with 1 over the square root of 2 times 0, 0, 1, 1, and
then all of that times u1. So 1 over the square root of 2
times the vector 0, 0, 1, 1. And so this is going
to be equal to v2, which is 0, 1, 1, 0. The square root of 2, let's
factor them out. So then you just get-- or kind
of reassociate them out. So then you get this is 1 over
the square root of 2 times 1 over the square root
of 2 is minus 1/2. You times-- what's the dot
product of these two guys? You get 0 times 0 plus 1 times
0, which is still 0, plus 1 times 1 plus 0 times 0. So you're just going to have
times 1 times this out here: 0, 0, 1, 1. I'll write that a little
bit neater. I'm getting careless. 1, 1. So this is just going to be
equal to 0, 1, 1, 0 minus-- 1/2 times 0 is 0. 1/2 times 0 is 0. Then I have two halves here. So y2 is equal to-- let's see,
0 minus 0 is 0, 1 minus 0 is 1, 1 minus 1/2 is 1/2, and then
0 minus 1/2 is minus 1/2. So V, we can now write as the
span of u1, y2, and v3. And this is progress. u1 is orthogonal, y2-- sorry,
u1 is normalized. It has length 1. Y2 is orthogonal to it or
they're orthogonal with respect to each other, but y2
still has not been normalized. So let me replace y2 with a
normalized version of it. The length of y2 is equal to
the square root of 0 plus 1 squared, which is 1, plus 1/2
squared, which is 1/4, plus minus 1/2 squared, which is
also 1/4, so plus 1/4. So this is 1 and 1/2. So it's equal to the
square root of 3/2. So let me define another
vector here. u2, which is equal to 1 over the
square root of 3/2, or we could say is the square root of
2/3, I'm just inverting it. It's 1 over the length of y2. So I'll just find the
reciprocal, so it's the square root 2 over 3 times y2, times
this guy right here, times 0, 1, 1/2, and minus 1/2. And so this span is going to be
the same thing as the span of u1, u2, and v3. And there's our second
basis vector. And we're making a
lot of progress. These guys are orthogonal with
respect to each other. They both have length 1. We just have to do something
about v3. And we do it the same way. Let's find a vector that is
orthogonal to these guys, and if I sum that vector to some
linear combination of these guys, I'm going to get
v3, and I'm going to call that vector y3. y3 is equal to v3 minus the
projection of v3 onto the subspace spanned by u1 and u2. So I could call that
subspace-- let me just write it here. The span of u1 and u2, just
for notation, I'm going to call it v2. So it's v3, and actually,
I don't even have to write that . Minus the projection
of v3 onto that. And what's that going to be? That's going to be v3 dot u1
times u1, times the vector u1. And actually let me just--
plus v3 dot u2 times the vector u2. Since this is an orthonormal
basis, the projection onto it, you just take the dot product
of v2 with each of their orthonormal basis vectors and
multiply them times the orthonormal basis vectors. We saw that several
videos ago. That's one of the neat things
about orthonormal bases. So what is this going
to be equal to? A little bit more computation
here. y3 is equal to v3, which
was up here. That's v3. v3 looks like this. It's 1, 1, 0, 0 minus
v3 dot u1. So this is minus v3,
1, 1, 0, 0, dot u1. So it's dot 1 over the square
root of 2 times 0, 0, 1, 1. That's u1-- so that's this part
right here-- times u1, so times 1 over the square root
of 2 times 0, 0, 1, 1. This piece right there is
this piece right there. And then we can distribute this
minus sign, so it's going to be plus. You know, we have a plus, but
there's this minus over here so we put a minus v3. Let me switch colors . Minus v3 , which is 1, 1 0, 0
dotted with u2, dotted with the square root of 2/3 times 0,
1, 1/2, minus 1/2 times u2, times the vector u2, times the
square root of 2/3, times the vector 0, 1, 1/2, minus 1/2. And what do we get? Let's calculate this. So we could take the-- so this
is going to be equal to the vector 1, 1, 0, 0, minus-- so
the 1 over the square root of 2 and the 1 over the square
root of 2, multiply them. You're going to get a 1/2. And then when you take the dot
product of these two, 1 times 0-- let's see, this is actually
all going to be, if you take the dot product of all
of these, then it actually gets 0, right? So this guy, v3, was actually
already orthogonal to u1. This will just go straight
to 0, which is nice. We don't have to have
a term right there. I took the dot product 1 times
0 plus 1 times 0 plus 0 times 1 plus 0 times 1,
all gets zeroed. So this whole term drops out. We can ignore it, which makes
our computation simpler. And then over here we have minus
the square root of 2/3 times the square root of 2/3
is just 2/3 times the dot product of these two guys. So that's 1 times 0, which is 0,
plus 1 times 1, which is 1, plus 0 times 1/2, which is 0,
plus 0 times minus 1/2, which is 0, so we just get a 1 there,
times the vector 0, 1, 1/2, minus 1/2. And then what do we get? We get-- this is the home
stretch-- 1, 1, 0, 0 minus 2/3 times all of these guys. So 2/3 time 0 is 0. 2/3 times 1 is 2/3. 2/3 times 1/2 is 1/3. And then 2/3 times minus
1/2 is minus 1/3. So then this is going to be
equal to 1 minus 0 is 1, 1 minus 2/3 is 1/3, 0 minus 1/3 is
minus 1/3, and then 0 minus minus 1/3 is positive 1/3. So this vector y3 is orthogonal
to these two other vectors, which is nice, but it
still hasn't been normalized. So we finally have to normalize
this guy, and then we're done. Then we have an orthonormal
basis. We'll have u1, u2, and
now we'll find u3. So the length of my vector y--
actually, let's do something even better. It'll simplify things
a little bit. Instead of a writing
y this way, I could scale up y, right? All I want is a vector that's
orthogonal to the other two that still spans
the same space. So I can scale this guy up. So I could say, I don't know,
let me call it y3-- let me call it y3 prime. And I'm just doing this to
ease the computation. I could just scale this guy
up, multiply him by 3. So what do I get? I probably should have done
it some of the other ones. 3, 1, minus 1, and 1. And so I can replace y3 with
this guy, and then I can just normalize this guy. It'll be a little bit easier. So the length of y3 prime that I
just defined is equal to the square root of 3 squared, which
is 9, plus 1 squared plus minus 1 squared plus 1
squared, which is equal to the square root of 12,
which is what? That's two square roots of 3. That is equal to 2 square
roots of 3, right? Square root of 4 times the
square root of 3, which is two square roots of 3. So now I can to find u3 as equal
to y3 times 1 over the length of y3, so it's equal to
1 over two square roots of 3 times the vector 3,
1, minus 1, and 1. And then we're done. If we have a basis, an
orthonormal basis would be this guy-- let me take the other
ones down here-- and these guys. All of these form-- let me bring
it all the way down. If I have a collection of these
three vectors, I now have an orthonormal basis for
V, these three right there. That set is an orthonormal basis
for my original subspace V that I started off with.