Determining the matrix of a linear transformation

• Jennifer1990
In summary, The matrix of D with respect to the standard basis of P3 is [0 1 0 0; 0 0 2 0; 0 0 0 3] if polynomials of degree 3 are included in P3 and polynomials of degree 2 are included in P2.
Jennifer1990

Homework Statement

Let D : P3--> P2 be differentiation of polyonimals. Determine the matrix of D with
respect to the standard basis of P3.

None

The Attempt at a Solution

I think D=[1 0 0; 0 1 0; 0 0 0]. This is from inspection though because I know that the resulting matrix must exclude the numbers in the last row of the matrix.
Could someone give me a hint on how to start this question?

Well first, do you know what is means by "the standard basis of P3"?

The standard basis is [1 0 0; 0 1 0 ; 0 0 1]

Jennifer1990 said:
The standard basis is [1 0 0; 0 1 0 ; 0 0 1]
How can that be? None of those are even elements of P3.

ohh wait wait , I meant [ 1 0 0 0; 0 1 0 0 ; 0 0 1 0; 0 0 0 1]

You have the dimension right, but those still aren't elements of P3.

um...then is it the set of different polynomials? {1, x, x^2, x^3}...i think

Jennifer1990 said:
{1, x, x^2, x^3}...i think
Yes, that's the standard basis for P3.

It's either that or the other way around. I think that P3, however, means polynomials of degree less than 3; so x^3 is excluded.

Now, what you do is you do the linear transformation on each of the basis vectors. This will produce a vector which is an element of P2. Say that U = P3 and V = P2, and D goes from U to V. Then L(u) = v, where u is in U and v is in V.

So you need to find: v1 = L(u1), v2 = L(u2), v3 = L(u3) (and v4 = L(u4), if you include x^3)

Then the matrix for D will have columns v1, v2, and v3 (, and v4 if you include x^3), in that order. Then if you take M, the matrix for D, and try

M*u = v, you'll find that v is a vector in P2 which represents the derivative of the polynomial u.

So
v1 = L(u1) = 0
v2 = L(u2) = 1
v3 = L(u3) = x
v4 = L(u4) = x^2

How am i suppose to rewrite this to represent the matrix D?...because this is now the standard basis for P2

Well, the derivative of x^3 isn't really x^2... it's 3x^2.

And, like I said, I believe that P2, P3, etc. means "the set of all polynomials of degree less than 2, 3, etc.". I might be wrong. That's how it was presented to me, however.

I'll do one part of it for you - so here we go.

U = P3 = [1, x, x^2], V = P2 = [1, x]

D(x^2) = 2x = transform(0, 2) wrt V

So the first column of M will be this vector, namely,

Code:
M = 0   ?   ?
2   ?   ?

To fill in the other question marks, do the same thing with the other basis vectors and put them in. Then you'll see that things like

M * transform(3, 7, 2) = transform(7, 4).

Does that make more sense?

Jennifer1990 said:
So
v1 = L(u1) = 0
v2 = L(u2) = 1
v3 = L(u3) = x
v4 = L(u4) = x^2

How am i suppose to rewrite this to represent the matrix D?...because this is now the standard basis for P2
What is L? You were supposed to be finding the coordinate matrix for D.

Do you know how to find the coordinates of a vector with respect to a basis?

To find the coordinates of a vector with respect to a basis, row reduce the vector with the new basis.

To:AUMathTutor
I think D works out to be [0 1 0 0 ; 0 0 2 0; 0 0 0 3] if we include polynomials of degree 3 in P3

I think that's right. I see I made a mistake in putting (0, 2) in the first column if x^2 comes last in my scheme.

I think that what you have is correct, if of course you allow polynomials of degree 3 in P3 and polynomials of degree 2 in P2.

1. What is a linear transformation?

A linear transformation is a mathematical function that maps vectors from one vector space to another while preserving the basic structure of the original vector space.

2. How do you determine the matrix of a linear transformation?

To determine the matrix of a linear transformation, you must first choose a basis for both the domain and range vector spaces. Then, the columns of the matrix will correspond to the images of the basis vectors in the range space under the linear transformation.

3. What is the significance of the matrix of a linear transformation?

The matrix of a linear transformation provides a concrete representation of the transformation and allows for easier computation and analysis of its properties. It also allows for the use of matrix operations to manipulate and understand the transformation.

4. Can a linear transformation have multiple matrices?

Yes, a linear transformation can have multiple matrices depending on the choice of bases for the vector spaces. However, all of these matrices will represent the same transformation.

5. How can the matrix of a linear transformation be used in practical applications?

The matrix of a linear transformation is widely used in fields such as computer graphics, engineering, and physics to represent and manipulate transformations in real-world scenarios. It is also used in solving systems of linear equations and in solving optimization problems.

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