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4.E: Exercises for Chapter 4

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    318
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    Calculational Exercises

    1. For each of the following sets, either show that the set is a vector space or explain why it is not a vector space.

    (a) The set \(\mathbb{R}\) of real numbers under the usual operations of addition and multiplication.

    (b) The set \(\{(x, 0)~ |~ x \in \mathbb{R}\}\) under the usual operations of addition and multiplication on \(\mathbb{R}^2.\)

    (c) The set \(\{(x, 1) ~|~ x \in \mathbb{R}\}\) under the usual operations of addition and multiplication on \(\mathbb{R}^2.\)

    (d) The set \(\{(x, 0) ~| ~x \in \mathbb{R}, x \geq 0\}\) under the usual operations of addition and multiplication on \(\mathbb{R}^2.\)

    (e) The set \(\{(x, 1)~ |~ x \in \mathbb{R}, x \geq 0\}\) under the usual operations of addition and multiplication on \(\mathbb{R}^2.\)

    (f) The set \(\left\{ \left[ \begin{array}{cc} a & a+b \\ a+b & a \end{array} \right] ~|~ a, b \in \mathbb{R} \right\} \) under the usual operations of addition and multiplication on \(\mathbb{R}^{2 \times 2}.\)

    (g) The set \(\left\{ \left[ \begin{array}{cc} a & a+b+1 \\ a+b & a \end{array} \right] ~|~ a, b \in \mathbb{R} \right\} \) under the usual operations of addition and multiplication on \(\mathbb{R}^{2 \times 2}.\)
    under the usual operations of addition

    2. Show that the space \(V = \{(x_1 , x_2 , x_3 ) \in \mathbb{F}^3 ~|~ x_1 + 2x_2 + 2x_3 = 0\}\) forms a vector space.

    3. For each of the following sets, either show that the set is a subspace of \(\cal{C}(\mathbb{R})\) or explain why it is not a subspace.

    (a) The set \(\{f \in \cal{C}(\mathbb{R}) ~|~ f (x) \leq 0, \forall x \in \mathbb{R}\}.\)

    (b) The set \(\{f \in \cal{C}(\mathbb{R}) ~|~ f(0) = 0\}. \)

    (c) The set \(\{f \in \cal{C}(\mathbb{R}) ~|~ f (0) = 2\}.\)

    (d) The set of all constant functions.

    (e) The set \(\{\alpha + \beta sin(x) ~|~ \alpha, \beta \in \mathbb{R}\}.\)

    4. Give an example of a nonempty subset \(U \subset \mathbb{R}^2\) such that \(U\) is closed under scalar multiplication but is not a subspace of \(\mathbb{R}^2.\)

    5. Let \(\mathbb{F}[z]\) denote the vector space of all polynomials having coefficient over \(\mathbb{F}\), and define \(U\) to be the subspace of \(\mathbb{F}[z]\) given by

    \[U = \{az^2 + bz^5 ~|~ a, b \in \mathbb{F}\}.\]

    Find a subspace \(W\) of \(\mathbb{F}[z]\) such that \(\mathbb{F}[z] = U \oplus W .\)

    Proof-Writing Exercises

    1. Let \(V\) be a vector space over \(\mathbb{F}\). Then, given \(a \in \mathbb{F}\) and \(v \in V\) such that \(av = 0\), prove that either \(a = 0\) or \(v = 0.\)

    2. Let \(V\) be a vector space over \(\mathbb{F},\) and suppose that \(W_1\) and \(W_2\) are subspaces of \(V.\)
    Prove that their intersection \(W_1 \cap W_2\) is also a subspace of \(V.\)

    3. Prove or give a counterexample to the following claim:
    Claim. Let \(V\) be a vector space over \(\mathbb{F},\) and suppose that \(W_1, W_2,\) and \(W_3\) are subspaces of \(V\) such that \(W_1 + W_3 = W_2 + W_3.\) Then \(W_1 = W_2.\)

    4. Prove or give a counterexample to the following claim:
    Claim. Let \(V\) be a vector space over \(\mathbb{F},\) and suppose that \(W_1 , W_2,\) and \(W_3\) are subspaces of \(V\) such that \(W_1 \oplus W_3 = W_2 \oplus W_3.\) Then \(W_1 = W_2.\)


    This page titled 4.E: Exercises for Chapter 4 is shared under a not declared license and was authored, remixed, and/or curated by Isaiah Lankham, Bruno Nachtergaele, & Anne Schilling.

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