Chapter 2: Problem 16
(a) Zeige, daß alle reellen Polynome, die höchstens den Grad 10 haben und eine Nullstelle bei \(x=1\), einen Vektorraum über \(\mathbb{R}\) bilden. Welche Dimension hat der Raum? Gib eine Basis dazu an!
Short Answer
Expert verified
The dimension is 10. The basis is \((x-1), (x-1)x, (x-1)x^2, \dots, (x-1)x^9\).
Step by step solution
01
- Define the Vector Space
Consider the set of all real polynomials of degree at most 10 that have a root at \(x = 1\). This set can be denoted by \(V = \{p(x) \,\|\, p(x) \text{ is a polynomial of degree } \leq 10 \text{ and } p(1) = 0\}\).
02
- Check for Vector Space Properties
Verify vector addition and scalar multiplication. For any polynomials \(p(x), q(x) \in V\), their sum \((p+q)(x) = p(x) + q(x)\) also has a root at \(x=1\). For any scalar \(c \in \mathbb{R}\) and polynomial \(p(x) \in V\), the product \((cp)(x) = c p(x)\) also has a root at \(x=1\). Thus, the set \(V\) is closed under vector addition and scalar multiplication.
03
- Check for Additive Identity and Inverses
The zero polynomial \(0(x)\), which is in \(V\) because \(0(1) = 0\), acts as the additive identity. For any polynomial \(p(x) \in V\), there exists an additive inverse \(-p(x)\) such that \(p(x) + (-p(x)) = 0(x)\), and \(-p(x)\) is in \(V\) since \(-p(1) = 0\).
04
- Basis and Dimension
Polynomials in \(V\) can be written as \(p(x) = (x-1)q(x)\) where \(q(x)\) has a degree of at most 9. Basis for \(V\) can be obtained from \((x-1)\), \((x-1)x\), \((x-1)x^2\), \dots\, \((x-1)x^9\), giving 10 elements. Thus, the dimension of the vector space \(V\) is 10.
05
- Final Basis Representation
The basis of \(V\) can be written as \[ \{(x-1), (x-1)x, (x-1)x^2, \dots, (x-1)x^9 \} \].
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
polynomial roots
A polynomial root, or zero, is a value that makes the polynomial equal to zero. For instance, if we have a polynomial \(p(x)\), and it has a root at \(x=1\), then \(p(1) = 0\). Each root corresponds to a factor of the polynomial. If \(1\) is a root, then \(x-1\) is a factor of the polynomial. For example, in the polynomial \(x^2 - 2x + 1\), rearranging it gives \((x-1)^2 = 0\), showing that \(x=1\) is a root.
vector space properties
A set is a vector space if it satisfies certain properties:
1. **Closure under addition and scalar multiplication**: For any two vectors \(u\) and \(v\) in the space, \(u + v\) must also be in the space. Similarly, for any scalar \(c\) and vector \(v\) in the space, \(c \, v\) must also be in the space.
2. **Additive identity**: There is a zero vector, usually \(0\), in the space such that for any vector \(v\), \(v + 0 = v\).
3. **Additive inverse**: For every vector \(v\), there is a vector \(-v\) such that \(v + (-v) = 0\).
In the context of polynomials, if we have polynomials that all have a root at \(x=1\), their sum or any scalar multiple will also have this root, showing closure under addition and scalar multiplication.
1. **Closure under addition and scalar multiplication**: For any two vectors \(u\) and \(v\) in the space, \(u + v\) must also be in the space. Similarly, for any scalar \(c\) and vector \(v\) in the space, \(c \, v\) must also be in the space.
2. **Additive identity**: There is a zero vector, usually \(0\), in the space such that for any vector \(v\), \(v + 0 = v\).
3. **Additive inverse**: For every vector \(v\), there is a vector \(-v\) such that \(v + (-v) = 0\).
In the context of polynomials, if we have polynomials that all have a root at \(x=1\), their sum or any scalar multiple will also have this root, showing closure under addition and scalar multiplication.
basis of a vector space
A basis of a vector space is a set of vectors that are linearly independent and span the whole space. Here, linear independence means none of the vectors in the basis can be written as a combination of the others. In the set of polynomials having a root at \(x=1\), a possible basis is \( \{(x-1), (x-1)x, (x-1)x^2, ..., (x-1)x^9\} \), because these are linearly independent and any polynomial in the space can be written as a combination of these basis vectors.
dimension of a vector space
The dimension of a vector space is the number of vectors in its basis. For the vector space of real polynomials of degree at most 10 with a root at \(x=1\), the dimension is 10. This is because we can create any polynomial in this space using a combination of the 10 basis polynomials: \((x-1), (x-1)x, (x-1)x^2, ..., (x-1)x^9\). Each polynomial is a unique element contributing to spanning the space.
real polynomials
Real polynomials are polynomials with real-number coefficients. They form the vector space in this context. Any polynomial of degree at most 10 with real coefficients fits into our vector space when it also has a root at \(x=1\). For instance, \(p(x) = x^3 - 3x^2 + 3x - 1\) is a real polynomial that fits into this vector space since it can be factored to show a root at 1, \((x-1)^3\). This space features properties such as closure under addition and scalar multiplication.