Consider the subset U ⊂ R3[x] defined as

[Karl Porter]

Homework Statement

Show that U is a subspace of R3[x]

Relevant Equations

U = {p(x) = a3x^3 + a2x^2 + a1x + a0 such that p(0) = 0 and p(−1) = 0}

so to show its a subspace (from definition) I need to prove its closed under addition and multiplication , contains 0 and for every w there is a -w? has it already been proven to contain 0 as p(-1)=0?
also I did sub in -1 and ended up with the equation a1+a3=a2+a0 but I don't know if that is relevant to solving this question?

 

[jbriggs444]

Let me try to get the notation clear first.

The sub-space you are talking about is the space of degree 3 (or lower) polynomials that have x=0 and x=-1 as roots.
The space you are talking about is the space of degree 3 (or lower) polynomials.

You are considering these as vector spaces, yes? So you want to prove closure under vector addition and upon multiplication by a scalar.

So, how would you go about proving either one of those for the sub-space?

 

[Karl Porter]

yea that's right
so for addition the sum of two vectors must belong to R3? when I substitute in -1 I get the equation a1+a3=a2+a0 but that leads me to ask how do i know a2+a0 is part of R3? is it a part of R3 because they are real numbers?

 

[jbriggs444]

yea that's right
so for addition the sum of two vectors must belong to R3? when I substitute in -1 I get the equation a1+a3=a2+a0 but that leads me to ask how do i know a2+a0 is part of R3? is it a part of R3 because they are real numbers?

Remember what you are trying to prove.

You are not trying to prove that the sum of two polynomials with roots at 0 and -1 is a polynomial. You already know that. It follows from the fact that third degree (or fewer) polynomials are a vector space in the first place.

You are trying to prove that the sum of two polynomials, each with roots at 0 and -1 is a polynomial with roots at 0 and -1.

 

[Karl Porter]

Remember what you are trying to prove.

You are not trying to prove that the sum of two polynomials with roots at 0 and -1 is a polynomial.

\You are trying to prove that the sum of two polynomials with roots at 0 and -1 is a polynomial with roots at 0 and -1.

so i would add the equation to itself but one would be x1 and the other x2 and see if its gets me a polynomial with those roots.
however i don't think i can solve that because it would have two unknowns

 

[jbriggs444]

so i would add the equation to itself but one would be x1 and the other x2 and see if its gets me a polynomial with those roots.
however i don't think i can solve that because it would have two unknowns

No. I do not think that you are getting it.

You are adding two polynomials to one another. Polynomials. Not values of the formal parameter x. You are not solving for anything. You are just checking to see if you get a result that has the right property. i.e. that it has roots at 0 and at -1.

The two polynomials would be, for instance, $(a_3, a_2, a_1, a_0)$ and $(b_3, b_2, b_1, b_0)$.

 

[Karl Porter]

No. I do not think that you are getting it.

You are adding two polynomials to one another. Polynomials. Not values of the formal parameter x. You are not solving for anything. You are just checking to see if you get a result that has the right property. i.e. that it has roots at 0 and at -1.

The two polynomials would be, for instance, $(a_3, a_2, a_1, a_0)$ and $(b_3, b_2, b_1, b_0)$.

but what happens to the x values?

 

[jbriggs444]

but what happens to the x values?

What is the sum of $a_3x^3 + a_2x^2 + a_1x + a_0$ and $b_3x^3 + b_2x^2 + b_1x + b_0$ ?

[One step at a time. We will worry about x in a moment.]

 

[Karl Porter]

x^3(a_3+b_3)+x^2(a_2+b_2)+x(a_1+b_1)+(a_0+b_0)

 

[jbriggs444]

x^3(a_3+b_3)+x^2(a_2+b_2)+x(a_1+b_1)+(a_0+b_0)

Perfect. Now two questions:

1. Is this a degree three (or fewer) polynomial?
2. Does it have roots at x=0 and at x=-1?

 

[Karl Porter]

Perfect. Now two questions:

1. Is this a degree three polynomial?
2. Does it have roots at x=0 and at x=-1?

yes degree 3
how do i know if it has those roots? i am left over with the constant values a_0+b_0?

 

[jbriggs444]

yes degree 3
how do i know if it has those roots? i am left over with the constant values a_0+b_0?

Suppose that the two polynomials that you just finished adding together were members of the sub-space.

That means that $a_3x^3 + a_2x^2 + a_1x + a_0 = 0$ when x=0 and when x=-1
That means that $b_3x^3 + b_2x^2 + b_1x + b_0 = 0$ when x=0 and when x=-1.

What do those facts then allow you to prove about the sum you just wrote down?

 

[Karl Porter]

a_0 =0 and b_0=0?

 

[jbriggs444]

a_0 =0 and b_0=0?

More basic.

You have two polynomials that are both zero at x=0. You add them together. What is their sum at x=0?
You have two polynomials that are both zero at x=1. You add them together. What is their sum at x=1?

 

[Karl Porter]

sum is 0

 

[jbriggs444]

sum is 0

Right. So is the sum an element of the sub-space?

1. Is it a polynomial?
2. It is zero at x=0 and at x=-1?

 

[Karl Porter]

Right. So is the sum a member of the sub-space?

1. Is it a polynomial?
2. It is zero at x=0 and at x=-1?

ok so the sum of the two polynomials will equal 0 which is part of the subspace. and since the two polynomials add to give a polynomial to degree 3 its part of R3

 

[jbriggs444]

ok so the sum of the two polynomials will equal 0 which is part of the subspace. and since the two polynomials add to give a polynomial to degree 3 its part of R3

Yes. Though I would state it a bit more carefully. The sum of the two polynomials is not the zero polynomial. The sum of the two polynomials is a polynomial which when evaluated at x=0 and at x=-1 yields a result of zero.

 

[Karl Porter]

Yes. Though I would state it a bit more carefully. The sum of the two polynomials is not the zero polynomial. The sum of the two polynomials is a polynomial which when evaluated at x=0 and at x=-1 yields a result of zero.

would method be the same for proving its closed under multiplication? wouldn't that give a polynomial to degree 6?

 

[jbriggs444]

would method be the same for proving its closed under multiplication? wouldn't that give a polynomial to degree 6?

Look back at post #2. We are not talking about multiplying two polynomials by each other. That is not one of the defined operations for a vector space.

The defined operation for a vector space is the multiplication of a vector by a scalar. i.e. multiplication of a polynomial by a real-valued constant.

On the other hand, perhaps you did not mean to be working with vector spaces. Perhaps you want to work in the ring of formal polynomials or in the ring of polynomial functions over the reals.

 

[Karl Porter]

Look back at post #2. We are not talking about multiplying two polynomials by each other. That is not one of the defined operations for a vector space.

The defined operation for a vector space is the multiplication of a vector by a scalar. i.e. multiplication of a polynomial by a real-valued constant.

righhhttt ok so that would give a polynomial to R3 but what about the zero root?
just same as before at x=0 and x=-1?

 

[jbriggs444]

righhhttt ok so that would give a polynomial to R3 but what about the zero root?
just same as before at x=0 and x=-1?

Assuming that we are working in the vector space of degree 3 polynomials [as opposed to the ring of formal polynomials or the ring of polynomial functions over the reals] then we can proceed as before.

Suppose that we have a real-valued constant $b$ and a polynomial $a_3x^3 + a_2x^2 + a_1x + a_0$ which is a member of the sub-space.

1. What is the product of that scalar and that polynomial?
2. Does that product evaluate to zero at x=-1 and at x=0? [editted to correct typo in original]

 

[Karl Porter]

K(a_3x^3+a_2x^2+a_1x+a_o)
and at x=0 x=-1 its gives 0

 

[jbriggs444]

K(a_3x^3+a_2x^2+a_1x+a_o)
and at x=0 x=-1 its gives 0

Good. So on that basis, is the product a member of the sub-space?

 

[Karl Porter]

OK. So on that basis, is the product a member of the sub-space?

yes!is that all that there is to prove to show its part of the vector space?

 

[jbriggs444]

yes!is that all that there is to prove to show its part of the vector space?

Well, we've proved closure over addition and under multiplication by a scalar.

Let me take a quick trip to Google and see what other axioms we need...

We need the existence of a zero element in the sub-space. This still needs to be proved.

Everything else, it seems that we inherit because we are a sub-space that is closed under addition and multiplication by a scalar.

 

[Karl Porter]

Well, we've proved closure over addition and under multiplication by a scalar.

Let me take a quick trip to Google and see what other axioms we need...

We need the existence of a zero element in the sub-space. This still needs to be proved.

Everything else, it seems that we inherit because we are a sub-space

so multiplying the polynomial by 0 would give a zero vector right?

 

[jbriggs444]

so multiplying the polynomial by 0 would give a zero vector right?

Yes. But you have to make sure that the zero vector is a member of the subspace...

... oh right. Since we've already proved closure under multiplication by a scalar, that one we get for free.

 

[Karl Porter]

the next part asked for the dimension of U
the lack of constants is throwing me off but I put in the values of -1 and 0 and I am left with a matrix
-1 1 -1 1 │ 0
0 0 0 0 │0
so wouldn't the dimension just be 1x4?

 

[jbriggs444]

the next part asked for the dimension of U
the lack of constants is throwing me off but I put in the values of -1 and 0 and I am left with a matrix
-1 1 -1 1 │ 0
0 0 0 0 │0
so wouldn't the dimension just be 1x4?

Can you justify the approach that you are taking here?

Personally, I would be thinking in terms of interpolating polynomials as a way to come up with an alternate basis for the vector space and for the sub-space.

You do understand the relationship between "dimension" and a "basis" for a vector space?