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: Suppose that the solutions of an equation \(A{\bf{x}} = {\bf{b}}\) are all of the form \({\bf{x}} = {x_{\bf{3}}}{\bf{u}} + {\bf{p}}\), where \({\bf{u}} = \left( {\begin{array}{*{20}{c}}{\bf{5}}\\{\bf{1}}\\{ - {\bf{2}}}\end{array}} \right)\) and \({\bf{p}} = \left( {\begin{array}{*{20}{c}}{\bf{1}}\\{ - {\bf{3}}}\\{\bf{4}}\end{array}} \right)\). Find points \({{\bf{v}}_{\bf{1}}}\) and \({{\bf{v}}_{\bf{2}}}\) such that the solution set of \(A{\bf{x}} = {\bf{b}}\) is \({\bf{aff}}\left\{ {{{\bf{v}}_{\bf{1}}},\,{{\bf{v}}_{\bf{2}}}} \right\}\).

Question 4: Repeat Exercise 2 where \(m\) is the minimum value of \(f\) on \(S\) instead of the maximum value.

Question: 17. Choose a set \(S\) of three points such that aff \(S\) is the plane in \({\mathbb{R}^3}\) whose equation is \({x_3} = 5\). Justify your work.

Explain why a cubic Bezier curve is completely determined by \({\mathop{\rm x}\nolimits} \left( 0 \right)\), \(x'\left( 0 \right)\), \({\mathop{\rm x}\nolimits} \left( 1 \right)\), and \(x'\left( 1 \right)\).

In Exercises 9 and 10, mark each statement True or False. Justify each answer.

10.a. If \(\left\{ {{{\mathop{\rm v}\nolimits} _1},...,{{\mathop{\rm v}\nolimits} _p}} \right\}\) is an affinely dependent set in \({\mathbb{R}^n}\), then the set \(\left\{ {{{\overline {\mathop{\rm v}\nolimits} }_1},...,{{\overline {\mathop{\rm v}\nolimits} }_p}} \right\}\) in \({\mathbb{R}^{n + 1}}\) of homogeneous forms may be linearly independent.

b. If \({{\mathop{\rm v}\nolimits} _1},{{\mathop{\rm v}\nolimits} _2},{{\mathop{\rm v}\nolimits} _3}\) and \({{\mathop{\rm v}\nolimits} _4}\) are in \({\mathbb{R}^3}\) and if the set \(\left\{ {{{\mathop{\rm v}\nolimits} _2} - {{\mathop{\rm v}\nolimits} _1},{{\mathop{\rm v}\nolimits} _3} - {{\mathop{\rm v}\nolimits} _1},{{\mathop{\rm v}\nolimits} _4} - {{\mathop{\rm v}\nolimits} _1}} \right\}\) is linearly independent, then \(\left\{ {{{\mathop{\rm v}\nolimits} _1},...,{{\mathop{\rm v}\nolimits} _4}} \right\}\) is affinely independent.

c. Given \(S = \left\{ {{{\mathop{\rm b}\nolimits} _1},...,{{\mathop{\rm b}\nolimits} _k}} \right\}\) in \({\mathbb{R}^n}\), each \({\bf{p}}\) in\({\mathop{\rm aff}\nolimits} S\) has a unique representation as an affine combination of \({{\mathop{\rm b}\nolimits} _1},...,{{\mathop{\rm b}\nolimits} _k}\).

d. When color information is specified at each vertex \({{\mathop{\rm v}\nolimits} _1},{{\mathop{\rm v}\nolimits} _2},{{\mathop{\rm v}\nolimits} _3}\) of a triangle in \({\mathbb{R}^3}\), then the color may be interpolated at a point p in \({\mathop{\rm aff}\nolimits} \left\{ {{{\mathop{\rm v}\nolimits} _1},...,{{\mathop{\rm v}\nolimits} _4}} \right\}\) using the barycentric coordinates of p.

e. If T is a triangle in \({\mathbb{R}^2}\) and if a point p is on edge of the triangle, then the barycentric coordinates of p (for this triangle) are not all positive.

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