# Linear Algebra - Representing Matrix

• Mumba
In summary, the map from R2 to R3 is a map from P2 to P3, with P2 being the function space of polynomials of degree <= 2, and P3 being the function space of polynomials of degree <= 3. Polynomial may be correct the way it has been written down, but I suggest you evaluate L at each of the three basis polynomials to see which gives you the matrix you are looking for.

## Homework Statement

The Question:

The map is given: $$L\rightarrow \Re_{2} \rightarrow \Re_{3}, p \rightarrow$$ p' + q*p , with q(x) = x.
Now i should find the representing matrix for L with respect to the bases {1+x, x+x2, 1+x2} for $$\Re_{2}$$ and {1,x,x+x2,1+x3} for $$\Re_{3}$$.

## The Attempt at a Solution

I don't know what i should do here ^^. But i tried to calculate it.
p is the polynomial, so i thought its like this:
p = a0(1+x) + a2(x+x^2) + a3(1+x^2)

So i can calculate $$p\rightarrow p' +q*p:$$
$$p\rightarrow a_{0}+a_{1} + 3a_{1}x + a_{0}x + 2a_{2}x^2 + 2a_{1}x^2 + 2a_{2}x^3$$

Then i just counted the coressponding values together, means
a0 ~ 1+x --> 1 + 1 + 0 + 0
a1 ~ x+x2 --> 1 + 3 + 2 + 0
a2 ~ 1+x2 --> 0 + 0 + 2 + 2

Puting this all together gives the repr. Matrix
| 1 1 0 |
| 1 3 0 |
| 0 2 2 |
| 0 0 2 |

Is this somehow correct or completely wrong? ^^
I'm missing the second base here..
Does anyone knows a good website (linear algebra 1) ? I have exam in little more than week and still a lot to learn.

Thx
Mumba

Mumba said:

## Homework Statement

The Question:

The map is given: $$L\rightarrow \Re_{2} \rightarrow \Re_{3}, p \rightarrow$$ p' + q*p , with q(x) = x.
Now i should find the representing matrix for L with respect to the bases {1+x, x+x2, 1+x2} for $$\Re_{2}$$ and {1,x,x+x2,1+x3} for $$\Re_{3}$$.

## The Attempt at a Solution

I don't know what i should do here ^^. But i tried to calculate it.
p is the polynomial, so i thought its like this:
p = a0(1+x) + a2(x+x^2) + a3(1+x^2)

So i can calculate $$p\rightarrow p' +q*p:$$
$$p\rightarrow a_{0}+a_{1} + 3a_{1}x + a_{0}x + 2a_{2}x^2 + 2a_{1}x^2 + 2a_{2}x^3$$

Then i just counted the coressponding values together, means
a0 ~ 1+x --> 1 + 1 + 0 + 0
a1 ~ x+x2 --> 1 + 3 + 2 + 0
a2 ~ 1+x2 --> 0 + 0 + 2 + 2

Puting this all together gives the repr. Matrix
| 1 1 0 |
| 1 3 0 |
| 0 2 2 |
| 0 0 2 |

Is this somehow correct or completely wrong? ^^
I'm missing the second base here..
Does anyone knows a good website (linear algebra 1) ? I have exam in little more than week and still a lot to learn.

Thx
Mumba
Your transformation is not a map from R2 to R3 - it's a map from P2 to P3, where P2 is the function space of polynomials of degree <= 2, and P3 is the function space of polynomials of degree <= 3.

yes, we called it R.
sorry i meant the same. A transformations from <=2 to <=3...

But R2 is not the same as P2, nor is R3 the same as P3. The dimension of R2 is 2, while the dimension of P2 is 3.

For this problem I would advise you to evaluate L at each of the three basis polynomials. Then write those three output polynomials in terms of the second basis. That should give you an idea of what the matrix for this transformation is.

Mark44 said:
For this problem I would advise you to evaluate L at each of the three basis polynomials.

But this is what i wanted to do ^^.
But i don't know how. I ve never seen this before...
Thats why i asked is that Polynomial correct the way i have written it down?

Maybe you can give me an example, let's say for 1+x...
What should i do with this?
Sorry but i really don't know...:(

Your formula is L(p) = p' + xp, so L(1 + x) = 1 + x(1 + x) = 1 + x + x^2. Do the same thing for the other two basis polynomials.

Thx!
then i get for:
L(x+x^2)=1+2x+x(x+^2)= 1 +2x+x^2+x^3
L(1+x^2)=2x+x(1+x^2)=3x+x^3

You said: write those three output polynomials in terms of the second basis.

so if i did this correct i get for
1+x --> 1 0 1 0
x+x^2 --> 0 1 1 1
1+x^2 --> -1 3 0 1

so my matrix is
1 0 -1
0 1 3
1 1 0
0 1 1

is this ok?

yeah its good
thx mate! ;)