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Find an example in \({\mathbb{R}^2}\) to show that equality need not hold in the statement of Exercise 23.

Short Answer

Expert verified

It is proved that \({\mathop{\rm aff}\nolimits} \left( {A \cup B} \right) \ne {\mathop{\rm aff}\nolimits} A \cup {\mathop{\rm aff}\nolimits} B\).

Step by step solution

01

Definition of the affine hull

The set of all affine combinations of points in a set\(S\)is called the affine hull(or affine span) of \(S\), denoted by \({\mathop{\rm aff}\nolimits} S\).

02

Find an example in \({\mathbb{R}^2}\) to show that equality need not hold in Exercise 23

RecallTheorem 2,whichstates that a set\(S\)is affineif and only if every affine combination of points of\(S\)lies in\(S\).

That is,\(S\)is affine if and only if\(S = {\mathop{\rm aff}\nolimits} S\).

Recall the equality holds in exercise 23 as shown below:

\(\left( {{\mathop{\rm aff}\nolimits} A} \right) \cup \left( {{\mathop{\rm aff}\nolimits} B} \right) \subset {\mathop{\rm aff}\nolimits} \left( {A \cup B} \right)\)

Let the sets\(A\)and\(B\), each of which contains one or two points.

Consider the set\(A\)containing the point\(\left( {0,0} \right)\), and the set\(B\)containing the point\(\left( {1,1} \right)\).

So,\(B\)contains two points\(\left( {0,0} \right)\)and\(\left( {1,1} \right)\). This implies that\({\mathop{\rm aff}\nolimits} \left( {A \cup B} \right)\)is in the line since it includes two points in lines.

According to theorem 2, if\(A\)are affine, then\(A = {\mathop{\rm aff}\nolimits} A\). Set B is an affine combination of set A since it contains\(\left( {1,1} \right)\).

The\({\mathop{\rm aff}\nolimits} B\)is the set contains two points, where\({\mathop{\rm aff}\nolimits} \left( {A \cup B} \right)\)through\(\left( {0,0} \right)\)but it is not same as\({\mathop{\rm aff}\nolimits} A \cup {\mathop{\rm aff}\nolimits} B\). This implies that\({\mathop{\rm aff}\nolimits} \left( {A \cup B} \right) \ne {\mathop{\rm aff}\nolimits} A \cup {\mathop{\rm aff}\nolimits} B\).

Thus, it is proved that \({\mathop{\rm aff}\nolimits} \left( {A \cup B} \right) \ne {\mathop{\rm aff}\nolimits} A \cup {\mathop{\rm aff}\nolimits} B\).

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Most popular questions from this chapter

Question: Let \({{\bf{v}}_{\bf{1}}} = \left( {\begin{array}{*{20}{c}}{\bf{1}}\\{\bf{0}}\\{\bf{3}}\\{\bf{0}}\end{array}} \right)\), \({{\bf{v}}_{\bf{2}}} = \left( {\begin{array}{*{20}{c}}{\bf{2}}\\{ - {\bf{1}}}\\{\bf{0}}\\{\bf{4}}\end{array}} \right)\), and \({{\bf{v}}_{\bf{2}}} = \left( {\begin{array}{*{20}{c}}{ - {\bf{1}}}\\{\bf{2}}\\{\bf{1}}\\{\bf{1}}\end{array}} \right)\)

\({{\bf{p}}_{\bf{1}}} = \left( {\begin{array}{*{20}{c}}{\bf{5}}\\{ - {\bf{3}}}\\{\bf{5}}\\{\bf{3}}\end{array}} \right)\) b. \({{\bf{p}}_{\bf{2}}} = \left( {\begin{array}{*{20}{c}}{ - {\bf{9}}}\\{{\bf{10}}}\\{\bf{9}}\\{ - {\bf{13}}}\end{array}} \right)\) c. \({{\bf{p}}_{\bf{3}}} = \left( {\begin{array}{*{20}{c}}{\bf{4}}\\{\bf{2}}\\{\bf{8}}\\{\bf{5}}\end{array}} \right)\)

and \(S = \left\{ {{{\bf{v}}_1},\,\,{{\bf{v}}_2},\,{{\bf{v}}_3}} \right\}\). It can be shown that S is linearly independent.

a. Is \({{\bf{p}}_{\bf{1}}}\) is span S? Is \({{\bf{p}}_{\bf{1}}}\) is \({\bf{aff}}\,S\)?

b. Is \({{\bf{p}}_{\bf{2}}}\) is span S? Is \({{\bf{p}}_{\bf{2}}}\) is \({\bf{aff}}\,S\)?

c. Is \({{\bf{p}}_{\bf{3}}}\) is span S? Is \({{\bf{p}}_{\bf{3}}}\) is \({\bf{aff}}\,S\)?

Question: Let \({\bf{p}} = \left( {\begin{array}{*{20}{c}}{\bf{1}}\\{ - {\bf{3}}}\\{\bf{1}}\\{\bf{2}}\end{array}} \right)\), \({\bf{n}} = \left( {\begin{array}{*{20}{c}}{\bf{2}}\\{\bf{1}}\\{\bf{5}}\\{ - {\bf{1}}}\end{array}} \right)\), \({{\bf{v}}_{\bf{1}}} = \left( {\begin{array}{*{20}{c}}{\bf{0}}\\{\bf{1}}\\{\bf{1}}\\{\bf{1}}\end{array}} \right)\), \({{\bf{v}}_{\bf{2}}} = \left( {\begin{array}{*{20}{c}}{ - {\bf{2}}}\\{\bf{0}}\\{\bf{1}}\\{\bf{3}}\end{array}} \right)\), and \({{\bf{v}}_{\bf{3}}} = \left( {\begin{array}{*{20}{c}}{\bf{1}}\\{\bf{4}}\\{\bf{0}}\\{\bf{4}}\end{array}} \right)\), and let H be the hyperplane in\({\mathbb{R}^{\bf{4}}}\) with normal n and passing through p. Which of the points \({{\bf{v}}_{\bf{1}}}\), \({{\bf{v}}_{\bf{2}}}\), and \({{\bf{v}}_{\bf{3}}}\) are on the same side of H as the origin, and which are not?

Question: 12. In Exercises 11 and 12, mark each statement True or False. Justify each answer.

a. If \(S = \left\{ {\bf{x}} \right\}\), then \({\rm{aff}}\,S\) is the empty set.

b. A set is affine if and only if it contains its affine hull.

c. A flat of dimension 1 is called a line.

d. A flat of dimension 2 is called a hyper plane.

e. A flat through the origin is a subspace.

Question: In Exercise 7, let Hbe the hyperplane through the listed points. (a) Find a vector n that is normal to the hyperplane. (b) Find a linear functional f and a real number d such that \(H = \left( {f:d} \right)\).

7. \(\left( {\begin{array}{*{20}{c}}{\bf{1}}\\{\bf{1}}\\{\bf{3}}\end{array}} \right),\left( {\begin{array}{*{20}{c}}{\bf{2}}\\{\bf{4}}\\{\bf{1}}\end{array}} \right),\left( {\begin{array}{*{20}{c}}{ - {\bf{1}}}\\{ - {\bf{2}}}\\{\bf{5}}\end{array}} \right)\)

Question: 29. Prove that the open ball \(B\left( {{\rm{p}},\delta } \right) = \left\{ {{\rm{x:}}\left\| {{\rm{x - p}}} \right\| < \delta } \right\}\)is a convex set. (Hint: Use the Triangle Inequality).

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